Mozhtratta

The Mozhtratta was a parasite that repeatedly encountered the Tenth Doctor and Heather.

Weakened, it hid away at Edinburgh University. Feeding on the residual vortex radiation, he planned his revenge and waited for Heather’s return. When she did reappear he captured her using students under his control and tied down to a ReCohesion Canon, he used the canon to drain the rest of the energy from her DNA. Unfortunately echoes of Heather’s life, her memories, manifested themselves and led the Doctor to her aid. When the canon was turned against Mozhtratta he “disappeared up his own atoms”, trapping him in his own DNA, which as the Doctor says is “frankly impossible”. (DWA: Dead-line)