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?
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.
- The description of ability damage and ability drain, which both come under the heading of ability score loss, says nothing about the two being equivalent.
- Ability drain’s section is next to, rather than under, the ability damage section; if it were under ability damage you could make an argument that it’s a special type of ability damage, but it isn’t. Instead they are next to each other, as two separate, parallel forms of ability score loss.
- The phrase “ability damage” is not mentioned anywhere within the ability drain section, so there’s also nothing explicitly linking the two, not even something akin to “ability drain is like ability damage,” (which on its own wouldn’t really be enough to say immunity to ability damage covers ability drain, but it would at least give the argument a leg to stand on).
Furthermore, there are monsters that list immunity to ability damage and ability drain separately, for example abominations which have
Abominations [...] are not subject to [...] ability drain, or ability damage.
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.
- In reality, ability burn is much more contentious here, since it says that is “a special form of ability damage.” It cannot be healed, but immunity would imply that it doesn’t need to be. But gaining immunity to ability burn would make Body Fuel into an extremely reliable and easy form of infinite power points.