If a creature has immunity to ability damage, does that extend to ability drain? For example, it's often suggested that the tarrasque can be rendered helpless by allips, using their Wisdom Drain ability. The tarrasque has immunity to ability damage, but doesn't mention ability drain, so will Wisdom Drain actually work on it?
[RPG] Does immunity to ability damage also give immunity to ability drain
No, it does not. To have immunity to ability drain, you need something that says you do. Something that only says you have immunity to ability damage just makes you immune to that, nothing else.1 In fact, there is really zero basis to think that they are the same.
Furthermore, there are monsters that list immunity to ability damage and ability drain separately, for example abominations which have
The thought that immunity to ability damage covers ability drain as well is nothing more than wishful thinking on the part of those who want to RAW-counter the anti-tarrasque allips. The rules don’t work that way: you cannot pretend things are the way you want in order to get the effect you want to achieve—that’s a houserule. Houserules are well and good (and, in fact, I’d argue they’re necessary for the game to run), but you cannot claim your houserule is what the books actually say no matter how much you wish it was what the book said. And, as a houserule, making ability damage immunity also cover ability drain is a huge change—it makes far more sense to just update the tarrasque’s immunities to also cover ability drain. Because immunity to ability drain is a lot rarer than immunity to ability damage, and that houserule would make it far, far more accessible.
Ultimately, though, I think that this vulnerability kind of reveals the grotesque flaws in the very concept of the tarrasque: it is, by necessity, a puzzle boss. Its stats and abilities are too strong to engage through anything other than a “trick,” whether that be ability drain or something else, but at the same time the tarrasque poses zero threat to a competent party whose levels are in the double digits, much less at 20th. Its exceptionally low Intelligence means it has zero ability to innovate or plot, so in the end it’s perfectly safe to just ignore it until you come up with some trick to beat it. In reality, the tarrasque is just not a well-made monster for those levels. Stick to dragons and fiends for that kind of epic threat.