Axos

Axos is an entity that came to Earth, supposedly in need of fuel. Its real motivations were to drain all energy from Earth.

Elements of Axos
Axos was composed of several parts, but the most noticable parts were the axonite and the axons.

Axonite
Axonite was the miracle element that the Axons traded with Mr Chinn for the energy to re-fuel their ship. Axonite was a living substance and they claimed that it could be used to enlarge the size of organisms. It was sent around the world to key scientific points, where it would drain energy evenly from those points. (DW: The Claws of Axos)

Axons
Though Axons had humanoid features, they to were part of the main Axos structure. They first took the form of golden skinned humanoids, but they could also take the form of large, tentacled creatures. They were capable of morphing between various humanoid shapes and teleporting themselves and others. (DW: The Claws of Axos)

History
Axos encountered the Master at an unknown point in time, at that time they captured his TARDIS, he in turn directed them toward Earth.

Following their arrival on Earth and attempt to start a 'feeding cycle' the Doctor placed their ship and all of Axos in a time loop. (DW: The Claws of Axos)