Rift Keystone

Rift Keystones were found in the Adventure Mode of Diablo III, obtained by defeating elite monsters. They could be used at the Nephalem Obelisk near the main camp of a respective act area. This would open a rift into a Nephalem Rift event. Keystones were replaced with Keystone Fragments.

A powerful mage could use this unstable artifact to open a portal to the alternate reality that birthed it.