Empathivore

Empathivores are creatures that feed on emotion.

Biology
The Empathivore encountered on Grått was a large, hairy, black creature with four eyes and multiple limbs.

An empathivore feeds off emotions including happiness, sadness, and fear. An empathivore can sense the strength of emotions and who is creating them. It grows as it absorbs more emotion, the Grått specimen growing from slightly larger than the Doctor to a giant that could easily pick up the TARDIS. If an empathivore consumes too much emotion, it explodes and dies.

History
An empathivore arrived on Grått at some piont and killed several locals. Known as the Mörkön, the monster became less dangerous after the Gråttites killed abandoned emotion. A Gråttite known as Grayla reintroduced emotions to Gråt and reawakened the Mörkön. The Doctor and Martha later visited the planet and defeated the Mörkön by overflowing it with emotion.