The Abbess

The Abbess was originally a human. Possessed by Gorgons, she served as their host on Earth for hundreds of years. She wanted to bring the rest of the Gorgons to Earth using a portal which resembled a well. She was turned to stone when Maria Jackson used a mirror to reflect the Gorgon's power back on itself.

Biography from the TARDIS Data Core article, licensed under CC-BY-SA