I didn't actually run an adventure like this, but I had a somewhat cool idea that happened to take shape, and figured someone else might want to use it, flesh it out and so on. Warning: There's some system specific stuff in here(D&D), sorry about that.
A bit of history. One of the campaign BBEG(big bad evil guy) is an evil lord trying to pass himself off as decent in an otherwise generally moral and good part of the world. The PC's know the truth, but it's too late to inform the proper authorities(his damage is already done). Long story short, the damage he's done is to take prisoner the good and noble lord responsible for a large army that the BBEG will soon face. The lord doesn't know he's a bad guy, so falls into his trap. The PC's learn of the lord's fate(essentially their patron), and predictably enough, it's high time to go kick some BBEG's butt. Fairly simple layout, but fun to play, good guys beat the bad guys, win the day, get the girl, blah blah blah.
Anyways, the way the find out is from the lord's youngest son, a noble himself, but quite inexperienced. He projects an air of knowing what he's doing, so the PCs let him tag along. Unfortunately, he *REALLY* doesn't know what he's doing, and ends up getting rather significantly killed in the very first encounter(something I hadn't exactly planned for).
Long story short, the PC's cart this body along with them until they reach the bad guy's lair, then leave the body up a tree while they deal with the bad guy. They rescue the good lord, and try to kill the BBEG. Unfortunately, the BBEG escapes with his trusty evil wizard accomplice, and runs away... and happens to find the son's body.
Here's where I didn't run the adventure I was thinking about(it didn't fit the campaign goals at the time):
What if the BBEG made the lord's son a zombie? Animate dead, and all that, made him some minion of evil, a sort of big old "screw you" to the good lord. The son couldn't be raised until he was no longer undead, so the lord wants to find and "save" his son so he can be at least buried properly, maybe even raised from the dead after his ordeal.
If the idea is to raise the son, the problem is that the clerics of the party are of high enough level to simply destroy the zombies as they turn them, making it somewhat more difficult to raise a body comprised of bits(I always imagined that destroying undead rather than turning them actually obliterated them). So, the clerics would not be able to destroy the undead in their quest to find the son, they would have to actually fight the undead they come across so they can retrieve the body of the son more or less intact.
It gives another chance to go after the BBEG, gives plenty of motive to hate the guy(something I really like is BBEGs that are truly despised, beyond simply their evilness). Plus which, then you can REALLY get some play out of zombies for experienced characters, where normally they are little more than stumbling blocks for a party to annihilate. The BBEG makes gobs of zombies for the PCs to fight through, and maybe uses the son zombie as a human shield or something.
There's lots of possibilities for this one, but unfortunately, our game didn't head in that direction. If you feel like running with this, "fleshing it out", be my guest.