These foul amulets are powered by the dark powers of necromancy. They bear the symbol of The Necromancer.
These devices allowed the lieutenants of the mountain top Temple of The Necromancer to command the undead there.
The party learned much later after they traveled across the desert and met more forces of The Horde that these amulets re-animate those who wear them.
There is speculation that they are scrying devices.