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I am a GM for both DnD 5e and Pathfinder 1e. I was thinking how some creatures in stories only die with a blow to a very specific spot and all hits anywhere else are completely nullified, and how common of a trope this is. I don't plan on using a creature like this but was brainstorming how would such a thing be implemented in game and I came up with two ideas:

  • They only take damage on critical hits (this is not a fun idea that I would ever implement, but it fits the concept)

  • Getting inspiration from werewolves who are generally only harmed by silver but in DnD have partial resistance only to other things. This is fair and good, but would still not fully implement the concept.

How do you implement a mechanic that would translate the idea of "this creature will die if hit in X spot and is invulnerable everywhere else"?

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    \$\begingroup\$ I'm VtC because this is subject to a lot of opinions, but also you've tagged this with both DnD 5e as well as Pathfinder 1e. The mechanical differences between these two are significant enough that a 'correct' answer is likely to be substantially different between the two. I recommend that you narrow your scope as much as possible (i.e. limit the question to something you actually want to implement) and keep it to a single system. \$\endgroup\$ Jun 14, 2022 at 17:41
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    \$\begingroup\$ @Pyrotechnical I think both are quire similar, not in all aspects but in many. All the answers given so far fit both systems \$\endgroup\$
    – TurtleTail
    Jun 14, 2022 at 18:09

9 Answers 9

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In a purely mechanical way of answering the question:

If you're playing on a battle map where the creature has a definitive size, you could consider making one of the tiles the creature occupies the "weak spot." Only an attack which can specifically target that tile the creature occupies could be considered as attacking the weak spot. (Once the players become aware of this weak spot, you'll presumably want to explain this mechanic explicitly since it's not a typical implementation of a creature). Optionally, other tiles the creature occupies could be considered various levels of cover to imply protective armor or the like.

For example, a 3x3 creature might have its center tile be the weak spot. A melee attack would have to either have reach to target that tile specifically or the attacker would otherwise find a way to occupy one of the creature's tiles so that they gain adjacency to that center tile. For ranged attacks, the weak spot tile could be targeted directly, but its "front" tile could be providing three-quarters or total cover to make a difficult shot, while its "rear" tile might be providing no cover.

For a weak spot not in a central tile of a creature, you might have to start considering facing rules, if only for that creature. For a 1x1 creature, the weak spot might simply be attacking from a specific angle (ex: "backstabbing").

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    \$\begingroup\$ This is an incredible idea, using a VTT (I use Foundry but roll20 should also work) one could make a 3x3 creature and make a highlighted square and even move it around if the players are not figuring it out. Like, the first attack goes normally, then describe a "core" moving out of there and highlight the square it stopped it. I thought there would be no good or fun way of doing it but now I definitely want to \$\endgroup\$
    – TurtleTail
    Jun 14, 2022 at 16:21
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Reskin normal combat

Follow the PHB that states on p. 196

Hit points represent a combination of physical and mental durability, the will to live, and luck. Creatures with more hit points are more difficult to kill. Those with fewer hit points are more fragile.

Have the hit points represent its invulnerability to anything but hitting their weak spot: their antenna, or their glowing hearstone, or whatever.

To hit the vulnerable part, you reduce their hp to zero. Until you get there, the attacks do no physical damage, while you are slashing their arms, legs, belly without hurting them. When you finaly hit them down to zero, the DM can describe how the stroke severs the antenna, or shatters the heartstone, and kills the creature.

Even if the characters know what they need to hit, until the hit points hit zero, the attacks that hit just mean the strike glanced off, they narrowly missed the right spot, etc.

The advantage of this approach is that it is in line with the way the combat system in 5e is designed, but you still can have the narrative you are looking for.

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    \$\begingroup\$ Taking this approach will surely end with players ceasing to try to attack the creature before getting it to zero. If my GM tells me "what you're doing right now appears to have no effect on this enemy", I will try something different, and if I exhaust all my options, I will flee. \$\endgroup\$
    – Vigil
    Jun 15, 2022 at 8:56
  • \$\begingroup\$ @Vigil: when we are fighting, we typcically ask if they are already bloodied, as per guidance, normally no creature will suffer physical damage until it has lost half its hits, so not seeing progress is not unusual. I agree that when you explicitly describe the hits not having a lasting effect, players will start trying different tactics. The DM can run with that and even have them flee to research their options, or can hint at the creature behaving to protect the vulnerable part, at which point the players will aim to hit it, and the battle turns into an effort to hit it. \$\endgroup\$ Jun 15, 2022 at 9:39
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    \$\begingroup\$ Normally in the games I play, if a creature is completely immune to the damage dealt, the GM will clue us into that by describing how the blow/spell etc. seemed totally ineffective. What you're proposing sounds like the GM would be saying pretty much the same thing, which would be interpreted by players as "we should do something else". I agree fleeing from such a creature is thematic - but the weird thing here is that when they "research" they will find the answer is not "use fire damage" or "only target this bit of the creature" but rather "you should have just kept hitting it normally". \$\endgroup\$
    – Vigil
    Jun 15, 2022 at 9:55
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    \$\begingroup\$ Players will not "aim to hit" a vulnerable part as 5e's system will have conditioned them to not ever ask to do that - they know that they are always trying to hit the best thing they can when they make an attack. \$\endgroup\$
    – Vigil
    Jun 15, 2022 at 9:56
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    \$\begingroup\$ Such a good question. Someone wants just once to have more than plain hp system. This answer says don't, just don't. Reskinning to make players think there's something to it, just to find out they were grinding hp bars like every other fight. That teaches players, don't use your mind. Keep swinging those swords and trust the encounters are balanced. \$\endgroup\$
    – Dor1000
    Jun 15, 2022 at 19:43
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You could do it more narratively. Give the creature regular, albeit abstract, hit points. This would be to reflect the idea that the heroes need to battle the creature of a while to tire it, and, ideally, discover that the weak point exists. Then have a special feature activate along the lines of "after the creature is reduced to zero hit points, then it's energy core becomes vulnerable. One hit to it (AC X) is required to finally vanquish the beast". This would result in less of a puzzle type of encounter, just more flavor on the encounter.

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  • \$\begingroup\$ This is a good compromise to address Pepijn's concerns. If the players solve the puzzle, they'll target the monster's weak point, short-circuit the encounter, and feel proud of their accomplishment. If they fail to solve it the puzzle, they'll be left thinking the weak-point was just flavor. Either way, the players will have a solid experience. \$\endgroup\$
    – Brian
    Jun 15, 2022 at 18:48
  • \$\begingroup\$ This is not as bad as groody's answer. Tiring is a good word: players can perceive their progress. There's precedent for killing blows to be extra epic, that's when you see beheading and other mayhem. Regenerating creatures often have a post-battle ritual to completely destroy them, like burning a troll. \$\endgroup\$
    – Dor1000
    Jun 15, 2022 at 20:10
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There is precedent for separate AC & hp for body parts

If you absolutely want to be able to separately attack different parts of a creature 5e style, you can take precedent from the Roper, which has separate hp and AC for body and tentacles:

  • Assign separate AC and hp to the vulnerable part and the rest of the body. The creature dies when you reduce its vulnerable body part to zero hp.

  • The rest of the body can have resistance or immunity against some or all types of damage.

  • You need to decide what happens if you recduce the rest of its body to zero, in case it is not immune to all damage: the creature dies, it is incapacitated while it recuperates, or it regrows a new body.

Attacking the vulnerable part needs nothing beyond stating that you want to attack it.

There are two issues with invulnerable monsters with an Achilles heel if directly directly implemented like this:

  • First, the encounter can either be a death trap or a joke, depending on the characters knowing about the weakness. If they do not know, it can make for a frustrating experience. If they do know, it can make the supposedly terrible monster ridiculosly easy to kill. This makes it difficult to have fun encounters with such creatures either way. See Pepijns answer for an in-depth exploration of this issue.

  • Second, the abstract combat system of D&D 5e is ill equipped to support a mechanism that needs a called shot. If charaters can target specific body parts, what is stopping them from hitting their humanoid opponents just in the head, and save all the hassle of long combat?

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For D&D 5e this makes me think of how vehicles, such as ships are implemented:

  • they are made of multiple components
  • components have individual AC and hit points
  • reducing hit points of certain components can have degrading effects
  • component can have damage threshold that needs to be exceeded on hit to do any damage

This can very well be applied to a (sizeable) creature design.

I think this also pushes for more flexible design than "all or nothing". Sure, the adventurers might have a plan how to reach and hit exactly that one component with low HP for devastating effect. Or maybe they are winging it and stuck with (possibly hopeless) frontal assault on sturdy high-to-ridiculously-high HP parts.

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DnD has always doubled down on the idea of player agency, PCs are generally supposed to feel powerful and their actions should have some impact. Because of this, invulnerability is rare and usually restricted to a damage type that many parties can overcome without much trouble.

Additionally, designing puzzle encounters is honestly pretty hard. Once you as the DM come up with a puzzle, you already know how to solve it and this makes it very hard to judge how hard or fair the puzzle actually is for the players. Puzzles that rely on a specific path to be taken often do not work well in play, anyone who has DMed knows players don't always do what you expect them to do. I would thus discourage the use of puzzles with one single solution, unless that solution can be made blatantly obvious through minimal trial.

A creature that is completely invulnerable except if you hit them in one specific spot leans towards this 'one solution' puzzle. It relies on players trying different things and eventually figuring out what is going on. The encounter will go as planned and be fun if the puzzle is figured out pretty soon into the fight. But as I said player don't always do what you think they will do because unlike you they don't know the planned outcome of the fight. If nobody figures out the weak spot, the encounter can very quickly become unfun or players might start to think they are not meant to fight it at all and will flee.

I am assuming that we keep the mechanics simple by giving the creature completely invulnerability unless players specifically state they target the weak spot, this is simple to play out and preserves the idea of this being a puzzle the players, not the characters, need to solve. I then personally have two main ways to help keep the puzzle fun:

  • Make the solution narratively obvious. If you keep mentioning a big glowing gem in every description of attacks not having effect, someone will likely start hitting it. Although this is technically just making the puzzle easier, I have found that figuring out unexpected puzzles is generally very satisfying for players regardless of their actual difficulty.
  • Include other elements into the fight that showcase the weakness, either at a predetermined time or as a backup plan if the players don't figure it out themselves. Perhaps an environmental hazard (traps, falling debris, etc) that happens to hit the weak spot, or a minor NPC who gets lucky (or unlucky, hitting his ally when missing his shot).

Although I think these could work, I still think single solution puzzles are generally not that great in play.

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    \$\begingroup\$ One small addition that isn't mentioned enough. PCs have some means to learn about the world. The creatures in it, their lairs and patrolled territory. If PCs know nothing about a monster before meeting it, you can heavily heavily signal the solutions to save them. But then there's no need to look after themselves and do their homework. Realistically there's so much knowledge and gossip PCs can collect. \$\endgroup\$
    – Dor1000
    Jun 15, 2022 at 20:19
  • \$\begingroup\$ @Dor1000 Very true! It does however rely on the players actually doing that :D. Rushing into an encounter should definitely make that encounter more difficult. I think in this specific instance it might become too difficult and potentially unfun, especially considering it's more of a puzzle than pure combat. But worst case you can always fall back on signaling the weakness during the fight, even though as you said it is a bit of a handout. Different tables will have different ways to play that work for them. \$\endgroup\$
    – anon
    Jun 16, 2022 at 11:44
  • \$\begingroup\$ Maybe my comment is too much. Players spending whole sessions making knowledge checks, canvasing for gossip, compiling accurate maps. Although that's what I would do as a char in that world. It makes little sense for an improvised game. \$\endgroup\$
    – Dor1000
    Jun 16, 2022 at 21:21
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I don't think there's much benefit in working out the mechanics in any real detail.

If the players don't know about the weak spot, combat should feel like stabbing a stone wall. Simply narrate the fight like you would if players tried to use their longsword to slash through a castle wall.

If the players know about the weak spot, then simply assume the characters aim for the weak spot. That's really not that different from fighting someone in plate mail. Combat should feel pretty much like normal combat. Maybe the creature has higher AC as if they were wearing armor, maybe the players have disadvantage on attacks.

The interesting thing about a creature with a weak point is not the combat mechanics, it's the social/espionage mission that goes into working out how to defeat the creature. Do the players withdraw from the fight and go talk to the sole survivor of the last group of adventurers to fight the monster. Do they study it while it hunts, or torture the local goblins until they reveal the information?

One thing to be aware of is the Chekov's gun principle. You might be a DM that describes everything in massive detail as a world building thing, or, more likely you follow the Chekov's gun principle - if you describe a gun sitting on a mantlepiece in the first act, it had better be fired in the second otherwise you shouldn't have described it". So, if you rarely give any details about monsters, but in this one case you go into a lot of detail about its eyestalks, your players will attach a lot more significance to the eyestalks than the characters have any right to, and a player might simply declare they are aiming for the eyestalks.

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  • \$\begingroup\$ Agree wholeheartedly with this analysis. It's only necessary to work out specific mechanics if the DM expects the players will have to discover the weak spot by accident, and (as others have noted) that is very likely to be a frustrating experience unless the DM heavily telegraphs the weak spot during combat. In any scenario other than dumb luck, everyone at the table will be aware of the fact that the party has learned that this creature has a weak spot, and it will be safe to assume that every attack attempts to target the weak spot. \$\endgroup\$
    – Tom
    Jun 16, 2022 at 1:15
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Multiple AC values

Or to put it another way, if a hit exceeds the AC by a certain amount it's hit the weak spot. You could have attacks deal less damage if they don't hit the weak spot, more damage if they DO hit the weak spot, or no blow can bring it below 0 unless it hits the weak spot. I'd suggest dropping the AC slightly, and having the "weak spot" AC be 5-10 higher. You might also consider having three values, AC, Weak spot AC and Weak Spot Targeted AC. So if for example your creature has a base AC of 13, a PC needs a 23 to hit it's weak spot, but as soon as someone does give a description of the attack being extra effective BECAUSE it hit the weak spot, and then attacks that are AIMED at the spot only need an 18 to hit it. This could also be gratifying if the party has done research on the monster before hand and KNOWS what the weak spot is (or maybe has learned a couple of possible weak spots only one of which is acurate).

I've run a similar concept recently, though it was a monster that would grab the PCs on a hit. If it beat their AC by 5 or more it also dealt crushing damage. Pathfinder already has Touch AC so it wouldn't be that much of a stretch to give a weak spot AC.

Another, similar possibility (you might want to skip this if you've got a barbarian in your party) is requiring a "reckless attack" to target the weak spot (but keeping the same AC either way) then it's a choice for the PCs rather than luck.

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    \$\begingroup\$ I've run monsters using both the "higher AC" and "reckless attack" approaches (successfully). For the former, I advise narrating the combat in greater detail than usual, so you can 1) narrate the "it staggers back as you hit the giant glowing bullseye" hits to hint that they should aim at it and 2) not make the hint too obvious by having it be the only "you hit this specific part" you narrate. A modification of the former approach that I've used is to also have hits near the vulnerable spot knock some armor loose (this should also be narrated), lowering both of the vulnerable spot ACs. \$\endgroup\$
    – Ray
    Jun 16, 2022 at 16:03
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There's a common type of enemy which is nigh-invulnerable except for a few small weak spots: A human wearing plate armor. Game mechanics reflect this by giving them a high AC.

You could use the same approach, by giving the enemy a high AC (higher than typical for the size) and low HP (representing the weakness). You don't even have to agonize over how much higher the AC should be: Just decide on the size of the weak spot, and use the existing rules for size-based AC bonuses.

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    Jun 16, 2022 at 18:34

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