The Ovu Mobani mask (which means Evil Eye) was an African piece with magical abilities.
Overstock from Joyce's Art Gallery causes her to decorate her bedroom with pieces from Africa. One piece in particular, a mask, turns out to be Ovu Mobani, a Nigerian Demon God of the Undead.
When Buffy was at a party organized by her friends and mother to celebrate Buffy's return from having ran away, the mask began animating animals and people as zombies.
Any zombie that puts on the mask will become the living embodiment of the Zombie God and be able to stun people with a flash from its "evil" eyes.