Nephalem Obelisks are found in every town in the Adventure Mode of Diablo III. The object is used to open Nephalem Rifts, as well as Realm of Trials and Greater Rifts with the corresponding Keystones: Rift Keystone Fragments, Greater Rift Keystones and Key of Trials. Each Keystone is consumed when used.
To activate an Obelisk, one must place the Keystone into the slot and press Accept. Note that Greater Rifts and Realm of Trials will teleport the player into the Rift immediately.
Once the rift closes, the obelisk is ready to be used again.