Diablo/Gharbad, the Weak

You will find this quest before Odgen's Sign quest (at dungeons level 4), but will complete it later.

Gharbad is a Clan unique demon that will not attack you, instead, he will beg you to not kill him in trade for a magic item. Just go to catacombs level 5 and kill some demons, then go back and talk to him. He will say that he's making you an magic item, and will beg again to not kill him. Repeat the same procedure two or three times, always talking with him. At some point he will turn against you and start attacking you. Just kill him and see what he drops.