Great Crystal
From Tardis Wiki, the free Doctor Who reference
The Great Crystal was created by the advanced civilisation on Manussa. They developed the crystal with their minds in zero gravity. Unfortunately their dark thoughts unintentionally created the Mara as well. After being defeated by the Fifth Doctor, the Mara sought out the Great Crystal in order to regain a physical body for itself. However, with the help of Dojjen, the Doctor used a small replica of the crystal to find a still point to destroy the Mara and its physical body. (TV: Snakedance)
