The Gravis

Gravis was leader of the Tractators who inhabited the planet Frontios. The Gravis was the only sapient Tractator, and controlled the others mentally.

Biography
Gravis appeared to confide in the Doctor, viewing him more favourably than the human colonists on Frontios. Gravis was more powerful and more intelligent than other Tractators. He was tricked by the Doctor into reassembling the Doctor's TARDIS. However the effort severly weakened him. The Doctor took him away from Frontios and left him on an uninhabited rocky planet called Kolkokron. (DW: Frontios)