Watchlar was an Eternine whose body merged with Jared Malahyde as part of their plan to drain Earth's energy to sustain their own dimension. Presenting themselves as humanity's future, Watchler and his people were able to manipulate Malahyde into completing the Utopian Engine, which would allegedly allow the Eternines to travel into the past on a large scale to help humanity evolve into them but in reality channeled energy from Earth to the Eternines by accelerating the age of all life on the planet.
When the Eighth Doctor and Isambard Kingdom Brunel disrupted the Engine before it could complete its operation, Watchlar was trapped inside the machine until the Doctor released him while trying to turn it off. Fortunately, the Doctor was able to link the TARDIS to the Utopian Engine and use both to turn time back to the moment before Watchlar contacted Malahyde, erasing these events from history. (PROSE: Reckless Engineering)