Axos

Axos was a composite creature, a scavenger entity which came to Earth, supposedly for fuel. Its real motivations were to drain all energy from Earth, with an ultimate plan to gain the secret of time travel, allowing it to feast anywhere in space and time.

Axos was first encountered and defeated by the 3rd Doctor during his period of exile on 1970s Earth. The Master attempted to ally himself with it, but later assisted the Doctor.

Since then the Doctor has encountered Axos again in various of his incarnations.