Mozhtratta

The Mozhtratta was a parasite who encountered the Tenth Doctor and Heather twice.

In 2009, Heather visited the Edinburgh catacombs, where she became separated from her friends after she witnessed a temporal echo of Great-Great-Great Grandmother McCrimmon being chased by the Mozhtratta. The Doctor found Heather shortly afterwards and explained what she had witnessed. Using his sonic screwdriver, the Doctor determined that Heather had traces of both vortex energy and the Mozhtratta in her DNA.

The Doctor determined that he needed Heather's help to deal with the Mozhtratta. He took her to his TARDIS - which was parked in the Edinburgh catacombs - and together they travelled to the Edinburgh High Street in 1815. Heather used the trace of the Mozhtratta in her DNA to weaken the Mozhtratta of this era while the Doctor helped Heather's ancestor overcome the Mozhtratta. (COMIC: The Chromosome Connection)

Weakened, he hid away at the University of Edinburgh. Feeding on the residual vortex radiation, he planned his revenge and waited for Heather's return. When she reappeared he captured her using students under his control and, tied down to a ReCohesion Cannon, he used the cannon 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 cannon was turned against Mozhtratta, he "disappeared up his own atoms", trapping him in his own DNA. (COMIC: Dead-line)