Doctor Daria Visteron was a human scientist from 35th century Earth who was recruited by Rasmus to work at the Deeptime Frontier. She was chosen because she had a historically-established reputation as a brilliant scientist and she was destined to disappear at the time when Rasmus recruited her, with her absence inspiring her daughter, also named Daria Visteron, to continue her research.
Although her recruitment was seen by the Time Lords as optional, she felt she had no choice to accept, and as a consequence held a grudge against them. With access to Vortex technology, Visteron scanned the Vortex for the Time Lord prison dimension containing the Ravenous, and released them. She claimed to only have wanted to see if the Time Lords could be scared too, and had no knowledge of what the Ravenous could do.