Anti-Key

A key to shadow-anchored gate - portals between the Material Plane, the Shadowfell, and potentially even the edge of the Negative Plane. This key is made of a cool, dark grey stone that shines like obsidian and constantly emits faint wisps of sand, as if blown about by a breeze even when the air around it is totally still.

Whoever wields the Anti-Key can, as an action, channel the power of it's sigil to open up an extra-planar door, similar to the Plane Shift spell.
Roll 1d100 - on a 1-75 you open up a door to a random location on the Shadowfell. On a 75-100, you open up a door to a random location on the Negative Plane. While on either the Shadowfell or the Negative Plane, the Key can also be used to return to the Material Plane at a random location determined by the DM. 

This Key has 1 charge per Long Rest.