Ygtjki

My DM has, for the sake of adding some light-hearted fun to our campaign, introduced a magic chicken that has 1HP but can only die due to the magic missile spell.

My question is, if we were to create this using templated rules text that does not simply say "immune to everything but magic missile," what all would be needed to cover absolutely EVERY cause of death other than that one spell? I have a list, below, of the features I have so far. What needs to be added to this list to cover everything?*

*(We can assume, though, that this chicken cannot gain class levels and cannot attune to magic items.)

• Immunity to all damage types except force damage


• Impossibly high armor class


• Immunity to exhaustion and incapacitation


• Infinite legendary resistance, and taking no damage when a passed saving throw would normally halve damage


• Critical immunity


• Immunity to aging and disease


• Immunity to spells cast by pyromancers (the Plane Shift: Kaladesh pyromancers have a level 18 ability that treats fire immunity as fire resistance).

Based on the wording used for the Rakshasa's Limited Magic Immunity and the Helmed Horror's Spell Immunity, it fits the template text of the rules to just write a feature that explicitly states the custom immunity. In other words, we don't need a fancy template, because the stat block designers already just make up whatever they need for a particular creature.

So the following works for your chicken and is just as rules-consistent as the Rakshasa and Helmed Horror:

Fowl Immunity. The chicken is immune to all harmful effects except magic missile.

It's conventional in the rules to use the phrase "harmful effects" to encompass damage, detrimental conditions, being killed, and so on. A related phrase that's frequently used is "harmful abilities, spells, or other magical effects," which is more verbose but captures the same information, so you can substitute that if you prefer the verbosity.

If Fowl Immunity isn't your kind of pun, try Bountiful Cluck, Poultry Excuse for an Immunity, Cheep Trick, or Unflappable.

There's always going to be some clever exception if you try to phrase it negatively like that. This is D&D, not a limited computer program. If you want something fancier than "immune to everything but magic missile", you could say:

Regeneration. The chicken regains 1 hit point at the start of its turn. If the chicken takes damage from the magic missile spell, this trait doesn't function at the start of the chicken's next turn. The chicken dies only if it starts its turn with 0 hit points and doesn't regenerate.

(Using the troll's ability as a template.)

In my experience, clever exceptions will always exist. If I were you I'd just give it "immunity to damage from sources other than Magic Missile" and play it RAI, accepting that some exceptions (like the pyromancer) will always theoretically exist, and you'll need to shut them down from time to time to enforce the chicken's special rule as you intend it to work.

It's a silly homebrew, and only for your table. As long as everyone knows what you mean, it should be fine. My DM writes a lot of homebrew stuff, and while I enjoy trying to translate it into as proper a syntax as I can, it's ultimately unnecessary. All that matters is that the rules are well-understood and enforced fairly, and that it's not ruining anyone's fun.

If I was going to implement this, though, I'd give the chicken something like 1000 AC and a Legendary Resistance (can choose to make a Saving Throw when you fail) that is at-will instead of having charges. An ability like the Rogue's Evasion, but adapted to work for all saving throws, is also necessary. I would go this route mostly for fun, to see the ways my party members try to break it (if they do). If you don't want it to be broken, a well-understood and accepted homebrew is the best way to do it.

There's always some weird exception that can bypass any particular written ability, one yet to be printed if not one already in existence. If you want to stick rigidly to ONLY magic missile, then you need to write that out explicitly, which I think is perfectly acceptable -- the rules for a creature can perfectly well refer to a particular spell.

However, I think this is a relevant question if you interpret it as, "What abilities would make a chicken effectively immune to everything except magic missile, even if there may be other, more obscure, similar effects".

Lets list off what you need to be protected against (please add to the list):

• Attacks (e.g. impossibly high AC and a special ability that ignores natural 20s)


• Failed saving throws (e.g. impossibly good saves)


• Damage from spells when you enter an area or start your turn there (not sure, some sort of teleport effect?)


• Damage from spells that happen immediately without any saving throw or attack roll (I can't find anything other than magic missile?)


• Damage on successful saves (Chicken needs Evasion, and the equivalent for other save types)


• Spells that kill you with no save (uh... Power Word Kill. Anything else? Not sure how to bypass that without calling it out explicitly. No damage resistance/immunity helps, and not much else would, unless you just say "immune to spells")

Other approaches

• Many ways of killing the chicken involve spells, most of which need to be targeted somehow. Make the chicken impossible to target somehow?


• Make it immune to every damage type other than Force. That gets round a lot of ways it might die. That might be best for flavour reasons, if other force spells did work. But you may be able to find extra abilities that protect it from the other possibilities, as well as .


• Have it be intangible, invisible and inaudable, auto-resurrecting, or similar. Although you'd need to find a reason why magic missile does affect it.

Tips for research

• Use google to search DnDBeyond, e.g: spells that mention damage but have no saving throw or attack roll
  

I think

• Immunity to all magical damage and effects except for those caused by the magic missile -spell


• Immunity to all physical damage


• Immunity to all conditions


• Doesn't age

should do the trick.