Lifesucker is a class three sword in terms of statistics. It can be gained as a loot from Conall mac Taidg's body when you have killed him during the quest Mac Taidg Must Die. The special property of the blade is that it does 10% extra damage against unarmoured humans.
This blade was used by Conall mac Taidg until you took it from him. He had no name for it, but it feels like it should have one, so now it's called Lifesucker.