CapnZapp
Legend
Maybe make generous application of Magic Jar: have a bunch of fake phylacteries around, feign death and then swap bodies with one of the player.
Agreed - this is exactly the kind of "bookish" smart aleck meta-spell a dick like Acecerak would use to torture the PCs with!That is golden right there.![]()

Only problem... apparently I'm not as smart as you, because what is the actual trick and how do you pull it off?
Best I can think of, is to have Contingency cast at Acecerak's death, that shunts his lifeforce into one of the phylacteries present at the scene.
If the party haven't already dumped them all into the lava, and/or are all protected against Evil and Good, then, just as the party breathes a sigh of relief, one PC after another is secretly forced into making the Magic Jar save. (It appears unlikely they can find, let alone destroy, that phylactery in just a few short rounds. The risk of everybody promptly running out of range is acceptable - though if he did spend a century here, the joke would be on him)
If and when it succeeds, one of the first actions Acecerak will do in his new body is - of course - to destroy the phylactery (with the trapped PC soul in it!). Perma-killing off player characters is, after all, on the table for end of campaign encounters!

Then, assuming he doesn't get to kill off one character at a time, he can go down in a blaze of glory, maybe taking another character with him... and reminding the remaining heroes that he will be back... not in 1d10 days, but to kill off the grandchildren of the heroes...

Is that even remotely how you intended Magic Jar to go down, or do I have an overactive imagination...?