We’ve all been there at one time or another: at some point, the party finds themselves fighting a regenerating creature, and the wizard casts disintegrate on it. The spell deals damage in excess of the enemy’s remaining hit points, and it goes down. But then the game grinds to a halt as everyone tries to figure out if its regeneration kicks in or not.
It’s easy to see why there’s a split of opinions in this regard. The spell says that if its damage reduces the creatures to 0 hit points or less, it’s reduced to a trace of fine dust. But the rules for regeneration say that, unless a particular damage type is specifically called out as bypassing a creature’s regeneration (e.g. acid and fire for trolls), then it’s actually dealing nonlethal damage, and so the creature’s hit points aren’t reduced to 0.
That, however, can seem counterintuitive, since it doesn’t explain why the creature hasn’t been reduced to dust (or if it’s regenerating back from dust, which seems like a bit much). But at the same time, the rules say what they say, and a straightforward reading makes it clear that regeneration trumps disintegration.
But is that really what the rules say?
Following the link above, the rules for regeneration note the following:
Regeneration does not restore hit points lost from starvation, thirst, or suffocation.
Attack forms that don’t deal hit point damage ignore regeneration.
However, those two sentences are presented as a single paragraph in the Monster Manual, and have an additional bit of text as well:
Attack forms that don’t deal hit point damage (for example, most poisons and disintegration) ignore regeneration. Regeneration also does not restore hit points lost from starvation, thirst, or suffocation.
Monster Manual v.3.0, page 10; Monster Manual v.3.5, page 314.
And there you have it, as the parenthetical listing in the above text notes that disintegration ignores regeneration. Why it was excised from the 3.5 SRD is unclear (especially since that notation was included in the 3.0 SRD), though the scouring of examples from the 3.5 SRD in general may have something to do with it.
Now, that disintegrate “do[es]n’t deal hit point damage” is a bit odd, since in 3.5 it does deal hit point damage; presumably, that’s a leftover reference to 3.0, when the spell only dealt hit point damage if the target made their saving throw (on a failed saving throw they were disintegrated completely, regardless of their hit points). However, it’s worth noting that the reference that disintegration overcame regeneration was never corrected in any errata issued for the 3.5 Monster Manual.
Given that the text is somewhat contradictory in that regard in 3.5 (specifically calling out disintegration as bypassing regeneration, but saying that it does so because it bypasses hit points, which it doesn’t), the debate is one that will likely never be completely resolved.
Intelligence Check 