Sentris was the 217th Coordinator of the Celestial Intervention Agency of Gallifrey. She was, in her own words, 'a cold-hearted killer'. She authorised the use of the Oubliette of Eternity on over 200 people in a year, dispersing them from history, meaning they weren't remembered. When Sentris found out what she had done on seeing information in a time-proof vault she dispersed herself and like her victims became a Neverperson, an inhabitant of the Antiverse, a Universe composed of Anti-Time. She devised a plan to release Anti-Time onto Gallifrey and destroy the Web of Time.
When the Eighth Doctor saved Charley Pollard from death on the R101 he created a paradox, giving the Neverpeople access to the Universe. The Neverpeople fed on time, and Anti-Time began to infect the Web of Time, causing anachronisms to seep into history. However Gallifrey was protected by the transduction barriers. The Neverpeople created the legend of Rassilon entering the Antiverse to battle Zagreus, luring Time Lords into the Antiverse. Sentris took the form of Charley Pollard when encountering the Time Lords. She made the Time Lords think a casket contained the remains of Rassilon, when it really contained Anti-Time. The Time Station containing the casket was set to explode when on Gallifrey, which would infect the Eye of Harmony and destroy the Web of Time. However the Doctor materialised his TARDIS round the ship before it exploded, saving Gallifrey. The Neverpeople on the ship were destroyed in the explosion. However the Doctor was infected by Anti-Time and became the monstrous Zagreus.