The Chapel of the Elders is a room in Phendrana Drifts, which houses the Arm Cannon expansion, the Wave Beam. When Samus first approaches the beam, multiple Baby Sheegoths are freed from four corner areas and attack her. After they are dispatched, Samus has her first encounter with a fully grown Sheegoth. This Sheegoth is noticeably larger then ones encountered later in the game.
After Samus defeats the Sheegoth, The Wave Beam will be available again, and Samus must use it to open the Purple Door leading back to the Chapel Tunnel.