Malachite called instructions through the entrance. The shield was unlocked by the careful tapping of selected stones with her hooves, in a specific order, with a certain rhythm. Celestia felt the stones drain a miniscule portion of her energy with each tap, to power the spell that would unlock the gate. After a few moments of instructions and tapping, there was a flickering over the mouth of the cave. The shield flared briefly, and then went dark. 