The Temperon was a legendary creature distantly related to the Chronovores. It could travel in time on its own power, its body being made of temperon particles. Continued exposure to temperon particles causes a slow, wasting death.
The Knights of Velyshaa attempted to capture the Temperon in order to conduct time travel experiments; however, the creature, preferring death to enslavement, tried to destroy them, creating a temporal distortion which froze the creature and the Knights in a single moment of time. The distortion eventually became known as the Kurgon Wonder.
An avatar of the Sirens of Time later manipulated the Doctor into freeing the Temperon and the Knights that were trapped along with it. This allowed the Knights to capture the creature and use it to power their time machines, albeit at the cost of their slow, painful deaths.
The Temperon brought the three incarnations of the Doctor that had been manipulated by the Sirens to Gallifrey. After warning them about the Sirens, the Doctors realized that the only way to defeat them was to free the creature, which they did. The Temperon then undid the changes they made to history and sacrificed its freedom to contain the Sirens in eternal struggle. (BFA: The Sirens of Time)