[RPG] When do monsters need to breathe


I've been doing some work on monster design, and stumbled on a weird problem with monsters needing to breathe. In my current monster write up, I don't intend for breathing to be something that is required. It's a magical creation, and it could survive with no oxygen if needed.

However, I've noticed that specific monsters have "Amphibious" if they can breathe both air and water (See Aboleth MM pg. 13). Also "Hold Breath" when they can hold their breath for long periods of time (See Plesiosaurus MM pg. 80).

Humans, which I'm positive have to breathe, have no statistics to indicate they do. Normally I'd say that breathing is inclusive, and a stat block will alter if that is the case (IE, they can breath water too). However, a good portion of undead/constructs shouldn't have to breathe (example, ghost) but there is no indication that they don't have to breathe.

When do I need to put something in to the stat block regarding when a monster can/can't breathe? (Assuming that the article would be published for distribution, so other people who did not design the creature might need to run it).

Best Answer

You don't need to make any special note in a creature's writeup unless you want to override the normal rules for how long a creature can hold its breath, or if it should have a special note of what it can do.

Those rules for suffocation are on page 183 of the PHB. They refer to a creature's Constitution score, so they apply seamlessly to monsters as well as PCs. Make sure your monsters have a sensible Con score for their concept, and how long they can hold their breath is taken care of.

Whether they need to breathe at all or not is generally left up to the DM's ability to interpret the writeup — obviously you don't think a ghost or zombie needs to breathe, so that's working as designed; and cases where it's less obvious (like: does a giant crab breathe underwater, or just hold its breath a long time?) are taken care of by the special abilities you note.

In general, give it an ability if it's non-obvious enough that one is necessary to avoid forcing a reader-DM to guess.