The type of undead is determined by the deceased magic-user's level / HD:
level 1 = skeleton
level 2 = zombie
level 3 = ghoul
level 4 = wight
level 5 = wraith
level 6 = specter
Some fun implications:
- low level parties have even more reason to protect the MU in case they find an enemy in their rear ranks
- common folk will be very leery of hosting a magic user in their community, lest the old boy die in his sleep one night and trigger a ghoul outbreak
- Raise Dead and Resurrection spells simply don't work on MUs (but can destroy undead)
- [my FAVORITE] ... the mid-level party who fights their way through the dungeon to face the evil wizard may not want to kill him and have to immediately go toe-to-toe with a specter, vampire or similar nasty. It may be far more prudent to capture the wicked fellow and bring him back to be imprisoned.
- the landscape is now dotted with haunted wizard towers
- clerics and the churches they serve may take a very dim view of those willing to unleash the curse of undeath upon the land in exchange for access to greater power during life
This doesn't apply to elves or other non-mortal spellcasters. Dead elves are simply dead.
Alas, this house rule doesn't fit any of my three current campaigns, being too grim for my elementary school crew, and off-theme for Dwimmermount (in which undeath and the quest for immortality are tightly defined), but I could see using it in a future game.
Have you ever put a similar spin on magic use or the undead?