Go to kill the evil lich, get captured and put at his/its mercy only to have it ask "Why are you bothering me?" Apparently it was/is a good wizard who got kind of absent minded as he died and sort of drifted off into lichdom without noticing. Since he's quite powerful, none of the various local monsters that he's geased into serving him have given him any trouble, nor have they pointed out the problem of his lichdom...
Play the lich as an absent minded old british gentleman, sort of surprised that anybody would want to kill him and having considerable trouble grasping the idea that he's a lich. A few accidental pats on the back while the players are held by some sort of spell should be amusing.
P.S. If you can't figure out how to set things up so a lich can capture and hold helpless a bunch of PCs, SHAME on you! Liches are something like 30th level M-U/Clerics, not to mention the hordes of followers, servants, summoned monsters and demons and elementals and the like...