How about we give it two possible abilities? One to energy drain all levels, killing the victim (and I guess normally causing them to re-animate as a wight), and another to shift it to wherever Khalk'ru lives? If you don't want both, I think I'd go for energy drain. Annihilation is ok, too, actually, if you have a strong preference.
Hmm… I considered the "shift to wherever Khalk'ru lives" option, but the original novel seems to indicate the body is destroyed.
I wouldn't include the standard 3E Energy Drain's "raised as a wight" rule since neither the OD&D or novel versions include it.
Also, draining ALL levels seems a bit extreme for a 15 HD creature. I'm tempted to reserve that to the full version and have the Aspect "only" drain a few energy levels.
In either case, I'd lean towards combining the two into a single ability.
Anyhows, here's a first stab at it:
Touch of Destruction [Aspect?] (Su): Any creature hit by an aspect of khalk'ru's tentacle attack must succeed at a DC X Will save or gain 1d4 negative energy levels. A victim who gains negative energy levels equal to their hit dice must succeed at a DC X Fortitude save or their corpse is utterly consumed as if affected by a
destruction spell. Creatures immune to energy drain and/or death effects are still subject to the
destruction effect it they fail both saving throws. The only way to restore life to a creature destroyed by khalk'ru's touch is to use
true resurrection, a carefully worded
wish spell followed by
resurrection, or
miracle. Victims who have merely had all their energy levels drained by khalk'ru may be restored to life by
resurrection. In either case,
restoration spells can be used to restore the lost levels.
Touch of Destruction [Full?] (Su): Any creature hit by khalk'ru's tentacle attack must succeed at a DC X Will save or gain negative energy levels equal to their hit dice. A victim who fails the Will save must then succeed at a DC X Fortitude save or their corpse is utterly consumed as if affected by a
destruction spell. Creatures immune to energy drain and/or death effects are still subject to the
destruction effect it they fail both saving throws. The only way to restore to life a creature destroyed by khalk'ru's touch is to use
true resurrection, a carefully worded
wish spell followed by
resurrection, or
miracle. Victims who have merely had all their energy levels drained by khalk'ru may be restored to life by
resurrection. In either case,
restoration spells can be used to restore the lost levels.