Simple question, are undead immune to exhaustion? My DM seemed to think so just based upon the fact they are undead. I said that unless their stat block specifically states they are immune to exhaustion, they're susceptible to it. Is there an official ruling?
A monster is immune to exhaustion only if its statblock says it is.
Notably, monster types to do not have rules of their own:
The game includes the following monster types, which have no rules of their own.
So there is no rule that all undead are immune to exhaustion, because that would be a rule about the monster type. So the only way to know if a particular undead creature is immune to exhaustion is to check its statblock. There are lots of undead that are immune to Exhaustion, however some undead, such as vampires, are not.
For comparison, the basic skeleton’s statblock reads:
Condition Immunities Exhaustion, Poisoned
But Exhaustion is notably absent from the basic zombie’s statblock:
Condition Immunities Poisoned
The monster’s statblock has everything you need to know, as stated in the introduction to the Monster Manual:
A monster’s statistics, sometimes referred to as its stat block, provide the essential information that you need to run the monster.
A monster’s statblock has the essential information you need to run that monster. There is no secret rule about undead you need to have found to know how to play the monster. The statblock has everything you need.