He isn't part of any mission, you can either kill him or let him sleep, he's the 7th prince of hell (Sloth) Located in Forsaken fortress behind a lvl 34 secret door (i belive), if you're planning on killing him, grind enough spirit res potions and HP potions in general, build up as much spirit res you can with your build just so it can be compatible since he does have very high chance of paralysis, if he locks on you and paralyzezs you, you're dead, no escape, except if you're doing the Warrior Class with Battle Rage.
-He drops very good items for 2-handed build warrior and Melee build Rogue. Demonclaw [2-handed greatsword ( 10-19 base damage +9 fire damage)] and Ghostfinger [Dirk (+5 shock damage +1 Armor)].