Obelisk

The Psionic Resonator channels the energy of living beings into a protective force that keeps closed the gateway between the Shadowshard and Rularuu's Cathedral of Pain. It is only strong so long as Rularuu's living servants are present. If they can be defeated, the obelisk becomes vulnerable and can be destroyed. Destroying all three Psionic Resonators at roughly the same time, is the only known way to open the gateway.