Osiris is the God of the Dead in anicent Eygpt. So ,he looks corpse-like, and can instant kill if your fortitude is less then 25.
If over 25, He has only one attack. Summoning Zombies.
Summons 6-18 level 25-95 zombies.
He drops a God Shard, used to craft Godly weapons and armors, and He is one of the 26 Gods. If in a level 150+ desert, you have a chance of finding him.