The Nucleus

The Nucleus was the control centre of the Swarm. It was a microscopic creature akin to virus with a crustacean-like appearance. In 5000, the Nucleus took control of the Fourth Doctor's brain, but was stopped by the Doctor "not thinking". It escaped through his tear ducts and was enlarged to human size by the TARDIS' relative dimensional stabiliser. When it tried hatching a hive of its Swarm on Titan Base, the Doctor ignited Titan's atmosphere, destroying the base along with the Nucleus. (DW: The Invisible Enemy)