Great Grey One is the leader and object of worship to the Teg-Du-Bhorez. She's sleeping in their fortress and all three Magic Stones are needed to awaken her. Her awakening also heralds apocalypse to the world, as was prophesied in the Tome of Ezekiel.
In 2002 Lara Croft, followed the hints of the Tome of Ezekiel and found all the Magic Stones. She then came to the Great Grey One's chamber and awoke, and killed her, to prevent any future awakenings.
The Great Grey One has several powers: She can throw energy daggers, energy balls, fireball, triple fireballs, electric balls, electric waves, call down lightning, reanimate Skeletons, and summon Shadow Monsters. She also can shapeshift, seen by her sleeping and awaken forms.