Koto

Koyo was a giant blue crystalline bounty hunter who had the special ability of absorbing air, heat and light into his body.

It was able to enter the TARDIS whilst it was still in flight after the Doctor unintentionally carried an escapee from a prison that he previously landed on. It intended to absorb the heart of the tardis in order to kill the Doctor as well as the escaped prisoner. (DWBIT: Cold Assassin)