Amulet of Darkness

This amulet is fashioned from a piece of black metal, with a symbol of a widening eye upon it. Whenever a creature casts a spell within 100 feet of the amulet, it begins to thrum subtly and glow with arcane energy of necrotic origin, as it steals a tiny amount of magical power from the spell. When it has collected enough power it will try to open a portal to another realm. At this point roll a D100. On a 1-40, it takes no effect and loses all power, having to start stealing magic again. On a 41-50, it deals 1 point of necrotic damage to anyone holding it, and likewise loses power. On a 50-99, it weakens the boundaries between our world and the Other Place, a ruined, dead, twisted landscape where darkness is king, allowing creatures in equivalent space in that dimension to attempt to break through. On a 100, it opens the rift to the Other Place, and the inhabitants can spill through at pleasure until the amulet is destroyed.