The Malignant

The Malignant was a force that appeared during the Last Great Time War after the War Doctor meddled with time and neutralised the Cyclors. (COMIC: The One) The Malignant killed fifty generations of the Overcast during the War. The Overcast blamed the War Doctor for this, accusing him of creating it.

The Malignant was still present on the Overcast homeworld nine hundred years later, when the Eleventh Doctor was brought to the planet to pay from his "crimes". (COMIC: The Then and the Now)