For the record, I’m an avid homebrewer and occasional free-lance third-party author. I have won awards and contests for my designs in D&D 3.5e and Pathfinder (Paizo’s closely-related spin-off). I think it’s important to know where I’m coming from, because ultimately a ton of what I’m going to say is stuff that I can’t easily back up—it reflects the perspective of someone who’s spent a ton of time on this stuff and I can’t transmit that experience to you through text. I’ll try to explain my thinking, but you may have to trust me on some of it.
Bottom line up front: LA doesn’t work
I have, at the time of this writing, read precisely one line from your write-up, and already know my answer from this alone:
And the answer is “No.” LA +2 is not and cannot be balanced, regardless of literally everything else going on. If it’s in line with official LA +2’s (which I will analyze for you—see below—but it won’t mean much), then it’s literally crippling. LA does terrible things to your HP, saves, skills, and feats, which are crucial elements of a creature’s power. Even more important, it delays progression through classes, which is a big deal for any classes that are worth a damn, because power grows super-linearly with level, that is, the level-up from 5th to 6th is worth more than the level-up from 2nd to 3rd, and so on.1
On the other hand, if it’s unlike official LA +2 options,2 and substantially powerful, you still have problems. Part of it is the fact that, again, not all levels are equal, so what’s “balanced” at one level won’t be at another. But more important than that is, even if it “balances,” the downsides of LA +2 are so immense that the power offerings have to be absurd—and that causes a hugely skewed character. Someone too good at some things and too weak to contribute at other things. Obviously, everyone has specialties, but LA almost by definition forces characters to be outside the “norm” in terms of things one is good at and things one is poor at. Again, hp, saves, feats, and skills are too weak, and level-appropriate class features are delayed to a point where they may not be appropriate any more, and then you have stuff that PCs shouldn’t have at that point, like gigantic ability scores, problematic movement modes, and so on.
LA simply cannot be balanced. I have put a lot of work into trying, and I have come to the conclusion it cannot be done. LA +1 can sometimes be borderline acceptable, but I don’t think anything greater than that is plausible.
Your one saving grace here is that you intend this to be defensive. That does help, because (I’m hoping) it will mitigate some of the biggest downsides of LA, the hp and saves. But there’s still plenty of other concerns going on.
For this section, I’ll actually be reading your write-up, not just focusing on the insurmountable problems of LA. This will not, however, change the real take-away here, which is that LA is simply not a good solution for the things it is supposed to solve, and cannot be made to work well.
- +2 Constitution, -2 Strength. Without the benefit of size, the mini-tarrasque is surprisingly weak. However, it still possesses exceptional fortitude.
- I scaled down the tarrasque by reversing the size-increase process in the MM and found that it actually had 8 Strength and a ton of Constitution.
I get Str 21, not 8—each size increase above Medium gives +8 Strength, and Colossal is 4 size categories above Medium, so that’s +24. The tarrasque has Strength 45, so subtracting 24 still leaves 21. Not sure where you got 8. (The Constitution, for the record, is 35−12, or 23, so it does end up just slightly higher than the Strength.)
More to the point, it just doesn’t capture what the tarrasque is about. Yes, Int 3 is not something anyone should be playing, but a small penalty to Intelligence would hardly be damning the character to stupidity.
Also somewhat unusual to have an LA +2 race and not have ability score modifiers that are net positives. That can have (serious) problems, but there’s a little wiggle room here.
I would go for something like +2 Strength, +4 Constitution, −2 Intelligence, myself, off the top of my head. Those numbers would be subject to change as I figured out the rest of the race. Constitution is the best ability modifier to have a bonus to when you’re ignoring what class features might be involved (it’s everyone’s second-favorite ability), but it doesn’t affect any skills beyond Concentration, so it kind of causes less skew.
Also, don’t take the Dungeon Master’s Guide too seriously on ability scores. They thought Strength was the most powerful ability score; it’s probably the weakest.
- Mini-tarrasque base land speed is 20 feet.
- Rush: Once per encounter, the normally slow-moving mini-tarrasque can move at a speed of 90 feet.
- Speed I kept at 20. [...] But 1/minute and 1/encounter seem almost identical unless you're going on an hour-long city rampage, so it seemed reasonable to make the 12-Con mini-tarrasque need a bit more rest between rushes than the 35-Con mega-tarrasque. The speed of the rush itself was nerfed down to the level of an aarakocra's flying speed, since they're also a listed Medium race that normally walks 20 feet per round.
In my (considerable) experience, encounters very rarely last more than a few rounds. I can’t remember ever having an encounter last a minute. Treating a “city rampage” all as one encounter is simply incorrect.
All of which leads to my response that “encounter” is a very nebulous thing in 3.5e. I would just go with the minute. Note that 20 feet 90% of the time and 90 feet 10% of the time averages out to 29 feet—very close to the typical Medium speed of 30 feet.
- Augmented Natural Weapon: The mini-tarrasque has sharp teeth and a powerful jaw. It can attack with its bite as a natural weapon, dealing piercing and bludgeoning damage equal to 1d6 plus its Strength modifier on a successful hit and threatening a critical hit on a natural attack roll of 19-20.
- Savage Species suggests [...] I think natural weapons are light, so I gave it basically the same stats as a shortsword.
Savage Species is arguably even worse than Dungeon Master’s Guide when it comes to balance. It wasn’t even originally intended to be a 3.5e product, but the start of a new d20 System game that got scrapped and shoved into 3.5e.
Anyway, yes, natural weapons are light, so yes, this is comparable to a short sword. Natural weapons have advantages and disadvantages relative to manufactured weapons, but ultimately this doesn’t really push power much at all.
- Blindsense: Though it lacks the finely-tuned scent ability of the full-sized tarrasque, the mini-tarrasque can use its ears and nose to notice the presence of things it cannot see. It has blindsense out to 15 feet.
- +1 LA for scent (which the tarrasque actually does have)? Naw, dawg. Let's take the minimum stated range for scent and turn it into blindsense. Something that conveys the tarrasque's extraordinary senses without actually being all that powerful.
Blindsense is way better than scent, and the tarrasque has scent, not blindsense. Just go with scent. Whoever thought scent was worth LA +1 was horribly wrong.
- Cause Fear: Once per day, when the mini-tarrasque charges or attacks a creature, it can affect that creature as though using the cause fear spell with a caster level equal to the mini-tarrasque's level and a save DC of 11 + the mini-tarrasque's Cha modifier. This is a supernatural mind-affecting fear effect.
- According to Savage Species, Frightful Presence is worth +1 LA. Without RHD, though, that is an abysmal trade. So I made the tarrasque's fear effect more like a quickened SLA, once a day. It's smaller, so obviously it's less intimidating. Still, probably enough to scare a common guard or what have you.
I’m more inclined to agree with Savage Species on this one—frightful presence is quite good. This cause fear effect, however, is near-worthless, because it targets only one thing, and that thing must have 5 HD or less, which means it does nothing to anything remotely threatening once you reach 7th level or so.
I would probably just have a “frightful charge” or whatever that forces a creature charged by the mini-tarrasque to make the Will save vs. shaken. I’d set the duration to 1d4 rounds, like demoralize is, and again have it end if the target gets 60 feet from the mini-tarrasque. I probably wouldn’t bother to limit it; the successful save granting 24-hour immunity would be sufficient.
That ability is good, no question, but it’s not out of line with a fancy racial feature. Especially since you can only frighten the thing you’re charging—that can be really awkward cuz often you don’t want them to run, you want them to stay where you can hurt them, and the things you do want to run, you don’t want to charge at and risk them not being afraid to turn on you. Still, for a charger, it could be effective, particularly combined with some Intimidate to get things up to frightened: charge to force them to run away, allowing you to charge again.
- Deflection: Despite not having a reflective carapace, the mini-tarrasque's flesh is still able to occasionally bounce away rays, lines, cones, and even magic missile spells. Whenever the mini-tarrasque is targeted by such an effect, it can roll a d6. On a 6, the mini-tarrasque ignores the spell or effect.
- Being totally immune to rays, lines, and cones felt way too good. That's a huge number of (sometimes very dangerous) attacks that the tarrasque can ignore. In 1e/2e, it was a d6 to decide if the attack was reflected or bounced harmlessly off. So I took that rate (about half of 3.5e tarrasque's reflection rate) and made it the odds of the attack bouncing off versus hitting you normally. I felt like that kept a good portion of the idea intact, without increasing mini-tarrasque's power too much?
First off, don’t call it deflection. Deflection is a thing, a type of AC bonus. Using that word for something totally different is confusing. Just call it carapace like the tarrasque does—obviously you aren’t going to give the full effect of the carapace, and “carapace” isn’t a general term like “deflection” is.
Secondly... it’s a ray/line/cone/magic missile-specific miss chance of about 17%. Miss chances are good, rays, lines, and cones are some of the most important stuff to avoid being hit by, but there’s still plenty of stuff it doesn’t protect against. So this is good, but probably not overwhelming. Fair enough, good design.
- Fast Healing: In contrast to the full-sized tarrasque's complete immortality, the mini-tarrasque can be slain in ordinary combat. However, its body still heals at an extraordinary rate, allowing it to regain 3 hit points at the beginning of each of its turns. Unlike to most creatures with fast healing, the mini-tarrasque also regrows lost limbs or body parts after 3d6 minutes. It can reattach a severed member instantly by holding it to the stump.
- Regeneration is the second of the tarrasque's most famous and important abilities, but the fact that nothing deals lethal damage to it seems like it'd be entirely busted for most of a campaign. I kept its ability to recover from damage (including dismemberment, albeit at troll speed rather than tarrasque speed), but going from regeneration to fast healing makes the mini-tarrasque actually killable. It also gives a +1 to LA (instead of +2), according to Savage Species.
I find it kind of appalling that Savage Species thought regeneration is only twice as good as fast healing. Regeneration is orders of magnitude superior.
Basically, fast healing is thoroughly mediocre. It’s like damage reduction/energy resistance that applies after the fact (so you could die before it applies), and worse, applies per-round instead of per-attack. Out of combat, it saves you some gold on wands of cure light wounds but that is really minor overall. But Wizards of the Coast consistently evaluated it as worth vastly more than it actually is—probably because they didn’t realize how much a wand of cure light wounds trivializes healing up between encounters. Technically, non-magical fast healing 3 is 50% better than the healing blood graft from Lords of Madness—which is supposedly worth an astounding 182,000 gp. Obviously, no one should be treating this as worth anywhere near a quarter million gold, much less potentially more than that.
Regeneration, on the other hand, is insane, and you were right to exclude it. Technically, that can be gotten for two feats—Toughness and Troll-Blooded—but Troll-Blooded is pretty frequently banned because two feats for regeneration isn’t really reasonable.
All that said, is fast healing 3 nice? Yes, it certainly is. It’s not worth LA +1, but I wouldn’t put it on LA +0 without some significant downsides to go with it. On an LA +2, it’s quite acceptable gravy. I like what you did with the limbs and so on—definitely a better call than going with regeneration. But note that lost limbs and the like are kind of a hypothetical concern in 3.5e—nothing in the game actually causes those to happen. Regeneration (and regeneration and ring of regeneration) are like the only references in the entire game to the concept, and those only cure it.
- Immunities: The mini-tarrasque is immune to ability damage and effects that would cause incurable wounds. Anything that would inflict ability drain deals ability damage instead. The mini-tarrasque automatically succeeds on saving throws made to avoid permanently losing one or more levels due to energy drain. If it would gain negative levels greater than or equal to its level, it instead gains negative levels up to a maximum of one less than its level.
- Like with being able to regrow/reattach body parts, I wanted the mini-tarrasque to be able to keep on trucking even against draining attacks. Almost no non-undead in the game has anything like this, so unsurprisingly Savage Species doesn't even have anything to say about it. But all abominations--tarrasque included--have immunity to these things, so I gave it a sort of "resistance to ability damage/drain and energy drain", based on how the horizon walker prestige class "resists" exhaustion. The energy drain immunity doesn't actually let it do that much that other races couldn't (since it still suffers penalties while fighting with drained energy), but it fits with the theme and lets it avoid two of the most frustrating things that can happen to a character.
Ability drain is a weird inclusion here because the tarrasque isn’t immune to that. That vulnerability is, in fact, quite notorious—it’s part of the reason why the CR 20 tarrasque can be fairly trivially defeated at 3rd level, because a 3rd-level wizard can cast command undead on some allips, against which the tarrasque can do nothing (they’re ethereal, and the tarrasque’s natural weapons count as epic weapons only for damage reduction, not striking ethereal foes) and has no defenses (they use a Wisdom-draining touch attack, so the allips basically cannot miss and can make the tarrasque unconscious with as few as four touches).
Rather, the tarrasque is immune to energy drain, and ability damage. Immunity to energy drain isn’t terribly rare—the LA +0 warforged race from Eberron Campaign Setting has that—but immunity to ability damage actually is. Not even the undead have that one (they instead do get immunity to ability drain). Constructs have immunity to both ability damage and drain, but the warforged’s living construct subtype restores vulnerability to both unless they take 5 levels of the warforged juggernaut prestige class (minimum ECL 10th). Warforged aside, there aren’t any reasonably-playable constructs out there.
But anyway, ability drain is worse than ability damage, so if the necropolitan (LA +0-ish template from Libris Mortis—it causes you to lose a level à la raise dead) can have immunity to ability drain, I think it’s no problem at all for an LA +2 race to have immunity to ability damage.
- Energy Resistance: The mini-tarrasque has resistance to fire 15.
- Again, immunity downgraded to resistance. Every version of the tarrasque is immune to fire (which is also in line with the fact that all abominations have an energy immunity, so it felt important to keep), but without the fire subtype (which would lead to cold vulnerability) that seemed like it might come with a hefty cost. Theoretically LA+1 for resistance to a single energy type of less than 20.
Fire damage is common, so resistance or immunity to it is nice. The number 15 isn’t really that great—while it’s rare to find resistance values much better than that, it’s not rare at all to find sources of fire damage that vastly exceed that, so subtracting 15 isn’t that big a win. Still, resistance 15 is at the high end of what you tend to see, and especially at lower levels it will be a pretty big chunk of savings. I wouldn’t take a race at LA +1 just because of fire resistance 15, but I wouldn’t put it on an LA +0 race either. On an LA +2 race, it’s fine.
- +2 racial bonus on saving throws against disease, poison, and energy drain. If it succeeds on its saving throw(s), the mini-tarrasque can overcome any disease or poison without the need for magical assistance.
- Keeping with the trend of "immunity to resistance", I gave mini-tarrasque dwarf-sized bonuses to saves against diseases and poisons, which for the normal tarrasque are immunities. I also gave it the ability to recover from magical diseases or whatever--basically just mummy rot, since that's something that the tarrasque specifically calls out as being immune to, and that keeps with the running idea of not getting slowed down by some incurable effect other than being killed.
Eh, poisons and disease are super-low-priority threats; you could safely give immunity to those here. Warforged have both. Conditional save bonuses tend to just get forgotten.
- +2 racial bonus on Listen and Spot checks.
- Tarrasque has +8 to Listen and Spot. Standard races (like elves) get +2 instead, so that's what mini-tarrasque gets, too.
Whatever. You could go higher at LA +2, but it’s not really a huge deal either way. Skill bonuses are cheap and plentiful.
- Automatic Languages: None. Bonus Languages: Abyssal, Celestial, Common, Infernal.
- By default, the tarrasque doesn't speak. Other abominations get Abyssal, Celestial, and Infernal, though, so now that the creature in question has more than 3 Intelligence, it has the potential to learn some languages. Not having Common available seems like it'd be pretty inconvenient for an intelligent avatar of destruction, so I threw that in as another bonus.
This is a terrible idea; you do not want to try to DM for a party that lacks a shared language. It’s got nothing to do with balance but this is a design mistake.
Also, the tarrasque isn’t an abomination; not just “technically,” but it really isn’t much of anything like them. In particular, the tarrasque is very much a Mundane creature—it’s not from the Outer Planes. As a result, the languages of the Outer Planes make a very strange choice.
- Favored Class: Fighter. A multiclass mini-tarrasque's fighter class does not count when determining whether it takes an experience point penalty for multiclassing
- Tarrasques fight. Fighters fight. I also considered barbarian, but felt like fighter was a more commonly-used favored class.
Favored class is meaningless: it only affects XP penalties, which almost no one uses (and absolutely no one should). Barbarian makes more sense to me than fighter but it’s really unimportant. Even in games that use XP penalties, they’re super-easy to avoid.
Conclusion on your design
The fast healing, carapace (“deflection”), and immunities are potent abilities that you would certainly expect LA for, and yeah, certainly in an official book, I would expect that LA to be higher than +1. How much higher Wizards of the Coast would put things at, I don’t know—they are not consistent about it, at all. But this isn’t as strong as saint, which is LA +2 (an overpowered LA +2, but nonetheless).
Where would I put it? I might well go with LA +1. LA +1 isn’t totally crippling, though it hurts a lot. But this gets a lot. If it was going to be LA +2, I’d expect higher ability scores, and probably more of the immunities intact. Energy drain, disease, and poison, for a certainty. Maybe fire, though that one is a little more dubious.
My approach: gun for LA +0
I have given up on LA in my games. What I do instead is come up with LA +0 versions of things when players want to play them. Doing that with the tarrasque is a tall order, but you have some really solid ideas here.
Constitution +2, Intelligence −4. Mini-tarrasques are durable, but often none-too-bright.
Monstrous Humanoid: The mini-tarrasque is a monstrous humanoid, and therefore is not subject to effects that target humanoids.
Land speed 20 feet.
Rush (Ex): A mini-tarrasque can, as a free action, increase its land movement speed by +30 feet for the rest of its turn. After doing so, it cannot use this ability again for 1 minute.
Darkvision 60 ft. (Ex): As a monstrous humanoid, the mini-tarrasque has darkvision out to 60 feet.
Carapace (Ex): A mini-tarrasque’s carapace functions as a suit of light armor, granting a +2 armor bonus to AC. It does not impose any maximum Dexterity bonus, armor check penalty, arcane spell failure chance, or speed penalty. The mini-tarrasque cannot wear another armor over their carapace, but it can be magically enhanced as if it were a suit of masterwork armor. The mini-tarrasque must be present the entire time the magic is being worked on, and if the carapace is magically enhanced, prevents the use of a magic robe the same as any magic armor would.
The carapace also provides a +4 deflection bonus against ray attacks, and a 50% chance for a magic missile to fail against the mini-tarrasque. If a mini-tarrasque makes a successful Reflex saving throw against a cone or line attack that normally inflicts half damage on a successful save (such as a red dragon’s fiery breath or a lightning bolt spell), it instead takes no damage, since its reflective carapace was able to harmlessly scatter the energy.
Fast healing 1 (Ex): The mini-tarrasque heals 1 hp at the beginning of each of its turns. Unlike to most creatures with fast healing, the mini-tarrasque also regrows lost limbs or body parts after 3d6 minutes. It can reattach a severed member instantly by holding it to the stump.
Immunities (Ex): Disease, energy drain, and poison. The mini-tarrasque is also immune to bleeding and incurable wounds.
Resistance to fire 5 (Ex).
Scent (Ex): The mini-tarrasque has a keen sense of smell.
Automatic Languages: Common; Bonus Languages: None.
Favored Class: Barbarian. A multiclass mini-tarrasque’s barbarian class does not count when determining whether it takes an experience point penalty for multiclassing.
Level Adjustment: +0
The net −2 on ability scores and the crappy-armor-you-can’t-replace are large drawbacks, and overall this race isn’t stronger than, say, warforged, which has similar armor, similar ability scores, stronger immunities as well as light fortification, but no fast healing or evasion. This mini-tarrasque is a strong race but not, I think, a too-strong race.
The warforged does offer feats to upgrade or eliminate the crappy armor, and this is probably a sensible idea. I would probably flavor these as improvements to the mini-tarrasque’s control over their healing, to allow them to bulk up or slim down as necessary.
This is not always true for every class at every set of levels, due to the inconsistent nature of class feature progressions. For example, getting your first 2nd-level spell at wizard 3rd is probably more significant than getting a second 3rd-level spell slot at wizard 6th.
Or it’s like saint, the one overpowered LA +2 option in the game.