Orcus does physical damage and some spell damage, both of which you should be ready for. He also carries a wand of death, although for any character with magic resistance, the main problem this poses is how to relieve him of it without him wasting too many charges.
If your attack merely causes damage, Orcus will normally teleport away to heal as soon as you get a substantial hit in. You need to prevent him from reacting to your attack. One way is to wield a cockatrice corpse or use a potion of paralysis. A mounted Knight can also easily joust Orcus with the lance repeatedly until dead; the stunning component of the joust prevents him from getting away.
Orcus in this form comes from Dungeons and Dragons. In turn inspired from a Roman god of the underworld.
He physically appears as a fat humanoid with a goat's head and legs, the horns of a ram, bat wings and a poisonous reptilian tail
Orcus, Prince of the Undead, has a ram's head and a poison
stinger. He is most feared, though, for his powerful magic
abilities. His wand causes death to those he chooses.