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).
Best Answer
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:
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.