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So I have a rather odd thing I am trying to do in my campaign. I am using spiders in a mine where my party is adventuring. Long story shortly the people before them basically have spider eggs laid inside them. My plan is to when the party reaches these poor people that it will trigger the heads of the people exploding and 20 or so tiny spiders come pouring out of the dead body. My question is, and I know there are spells to enlarge creatures and what not but what I want to do is advance the spiders to the next size category as sort of an innate ability or programmed event or something. So the tiny spiders attack the party and in like 5 rounds say if not killed they grow to the next size category.

Does anyone have a way to do this that isn't totally ridiculous? I realize that to many my plan already is ridiculous but any helpful suggestions would be greatly appreciated.

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About the timing

First, five rounds is much, much too long. A typical combat is decided in 2-3 rounds; a 5-round combat is really long. Maybe with a lot of mooks to mop up things could drag on that long, particularly if their appearance is staggered so they can’t all be caught in one area blast, but that doesn’t generally make for a very good fight.

In fact, so much happens in a single turn, to say nothing of an entire round, that giving the players even a full round of combat before the monsters become “real” is quite a substantial advantage for them. I would go with just one round.

About the balancing

Second, this kind of thing isn’t really a matter of rules. I mean, I suppose they could have written up something for this, but I’m pretty sure they haven’t—it’d be pretty surprising if they had. You’re going to have to just do it as a matter of DM fiat. Which is fine, it just means a little more work on your part getting the numbers right.

Specifically, if you have a bunch of Tiny monstrous spiders at CR ¼, but after one round they balloon into Small monstrous spiders at CR ½, whether you should be counting them as CR ¼ or CR ½ depends a lot on how many there are, and how many the party can eliminate in one round. If the party can and will blast them all immediately—whether that’s an archer with a ton of arrows per round, a warrior with Whirlwind Attack or something similar, or a mage with burning hands or fireball or whatever and willing to use it, most of them are going to be gone before they actually become CR ½. Even if not, you figure they party is probably going to go through roughly one each, so if you only have 4, once again, all gone. If you have 20, and they only go through 4, they have 16 left that are now Small—each CR ½—and maybe your encounter is looking more like it’s at that CR.

If you plan on having them grow again in round three, I’d just ignore that. Some might still linger by that point, but if the party has their act together it shouldn’t be many.

So anyway, you basically have to imagine how the players are going to react, and scale it accordingly. You have to think about what their capabilities are, and whether or not they’re likely to respond to this threat quickly. If their area-damage options are limited, maybe they won’t use them on Tiny spiders—maybe it’s OK if they trivialize the encounter by spending a spell slot or whatever. If their area damage options aren’t limited, and you assume they’ll use them, then maybe you want the encounter to be bigger to make up for that. Or maybe you want to leave some people still OK, so they can’t just blast all the spiders. Up to you, really, but the more complicating factors you throw in—the less likely they are to be able to mop up the spiders in a timely fashion—the more this is looking like the higher CR option than the lower.

Also, for the record, I chose the Tiny and Small spiders because you mentioned Tiny in your question. Scaling this up—having the spiders start Medium and become Large, for example—should work out OK enough, up to a limit anyway. But then the spiders are presumably limited to being smaller than their hosts, so it may not matter., that’s really not so different from any other case—CRs and player character power levels are both so wildly variant that CR is terribly unreliable. No matter what you do, you have to carefully consider how your party is going to deal with this encounter if you really want to know how it’s going to go.

About how this compares to the rest of the game

Third, that’s really not so different from any other case—CRs and player character power levels are both so wildly variant that CR is terribly unreliable. No matter what you do, you have to carefully consider how your party is going to deal with this encounter if you really want to know how it’s going to go.

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    \$\begingroup\$ Thank you very much for the response. I appreciate all the information. \$\endgroup\$ – Jonathan Jan 9 at 10:44
  • \$\begingroup\$ @nijineko Compete by writing great answers, not by arguing in comments that yours is better. \$\endgroup\$ – SevenSidedDie Jan 10 at 16:50
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    \$\begingroup\$ @sevensideddie why side with someone who didn't answer the question, versus someone who did? Kryan's answer doesn't meet the site criteria. \$\endgroup\$ – nijineko Jan 10 at 17:05
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    \$\begingroup\$ @nijineko I’m not taking sides and haven’t even read the answers, I’m using my Mod Voice to explain to one that they’re misusing their comment privileges. (If there is a problem with an answer such that it needs to be removed, flags and meta are provided. Comment arguments are not the appropriate course of action.) \$\endgroup\$ – SevenSidedDie Jan 10 at 17:16
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    \$\begingroup\$ Thank you for making me aware of flags, first time hearing about that. \$\endgroup\$ – nijineko Jan 11 at 2:09
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Perhaps having them form into a spider swarm works better?

From this site:

Spiders will lay between 2 and 1000 eggs, depending on the species.

So when they hatch, have all the spiders start to form a swarm. After the 1st round, the spider swarm forms. Or, a full swarm spawns from the head of each NPC? You didnt say what level your party is, so a single spider swarm can be a pretty tough battle for a starting out party that doesn't know how to use a torch against swarms since they are immune to melee damage. You still have to worry about AoE damage from spell casters, but they always ruin the fun with their disintegrate and fireball spells(tic).

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  • \$\begingroup\$ Thank you for the information that is very helpful and I appreciate it. The party just made level 3 and there are 4 in the party. \$\endgroup\$ – Jonathan Jan 11 at 1:57
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Mechanically speaking, it sounds like what you want is the spawn ability plus a Wu Jen spell.

With spawning, a creature becomes infected from a monster ability or wound, and if not treated appropriately within a time limit, the creature converts into more of the creature that inflicted them in the first place.

A spawn is sometimes fully grown, sometimes an immature or partial version of the monster (like vampires), and is usually under the control of the original monster that infected the creature.

This gory details of exploding heads (see Vargouille in MM1) are just fluff, not really important mechanically, but important to the look and feel, descriptions, story, and emotional aspects. So you can make them work however you want without too much impact to the game.

Regarding swift size changes, there is a Wu Jen spell called Giant Size, that should do the trick nicely. If they have that as a spell-like ability a limited number of times a day, you have your giant spiders.

Note: the Wu Jen spell skips the middle and small size categories, duplicating an effect commonly found in Asian mythology, so this may not be a perfect fit, but boy will it surprise your party!

Bonus points if some of them are G.I.F.T.S.

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  • \$\begingroup\$ Hmmm I will have to look into the Wu Jen spell. I am more thinking a magically manipulated species of spider who's hatchlings act differently and grow very quickly but your suggestions do help and I appreciate it thank you. \$\endgroup\$ – Jonathan Jan 11 at 1:59

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