The Malignant

The Malignant was a force that appeared during the Last Great Time War to kill the Overcast. The Overcast blamed the War Doctor for the Malignant killing fifty generations of their people, 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)