The Gravis

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

Biography
The Gravis appeared to confide in the Fifth Doctor, viewing him more favourably than the human colonists on Frontios. The 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 weakened him severely. The Doctor took him away from Frontios and left him on an uninhabited, rocky planet called Kolkokron. (DW: Frontios)