There's a battle going on between a good lich and one or more evil liches. The players have to protect a town that's caught in the crossfire. The lich need not even appear in the campaign; you could just have dark noxious clouds blotting out the sun, undead armies marching back and forth, dragons eating the livestock, and other bits of large- scale magical fallout. Or, if you want to bring the lich in personally, you could send the party on a quest to plead with the lich to stop the war, or to fight elsewhere.