The Nucleus

The Nucleus was the control centre of the Swarm. It was a microscopic creature with a crustacean-like appearance. With the use of the TARDIS' relative dimensional stabiliser, in 5000, Professor Marius brought it to human size. When it tried hatching a hive of its Swarm on Titan Base, the Fourth Doctor ignited Titan's atmosphere, destroying the base along with the Nucleus. (DW: The Invisible Enemy)