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 Greater Rifts with the corresponding Greater Rift Keystone. Each Keystone is consumed when used.
To activate an Obelisk, one must choose the desired Rift type and difficulty and press Accept. Note that Greater Rifts will teleport the player into the Rift immediately.
Once the rift closes, the obelisk is ready to be used again.