When it comes to the undead, there are few monsters that I like more than the lich.
The lich is an undead wizard that typically specializes in necromancy. Liches are notoriously difficult to destroy, as they have a phylactery that must also be tracked down and destroyed. The process of finding that phylactery could make a quest all unto itself since liches tend to be a bit paranoid that people are going to destroy their soul-holding containers that guarantee the lich eternal life.
A lich’s desire to not die and subvert the natural order is an understandable and very human one. Their obsession often leads to madness, and makes them prime candidates for the position of big, bad boss guy.