The First

The First was the very first sentient lifeform in the universe. She saw herself as the mother of all living things. She slept for billions of years, waking to find her children fighting, causing wars and sadness. This caused her to create a Reality Virus, to wipe out all creation and start again.

The Thirteenth Doctor and the Player stopped her by sealing her in a time loop. (NOTVALID: The Edge of Time)

After one of her defeats in the time loop, the First's virus was re-purposed by the Cybermen of the CyberReaper. (NOTVALID: The Edge of Reality)