What about.... Your characters work for an evil necromancer, who sent them out on a mission to find the last spell component, the evil necromancer needed to complete his evil "control all undead" spell. And if they are good, you can let them find out how evil the necromancer is, and then turn the mission around. So now they have to KILL the evil necromancer.
Feel free to change, whatever you want to.
Btw, did you notice that I used the word "evil" 5 times.