Kronos was the result of a forbidden union, made unlawful by the Ancient Convenant, of a Chronovore, Prometheus, and the Eternal known as Electra. (PDA: The Quantum Archangel) He described himself as "beyond good and evil".
In the 1970s, the Master used a crystal named after Kronos to cause him to manifest so that (he thought) he might bind Kronos to his will. The Master failed, and, returned back in time to the later days of Atlantis, Kronos ruined the city. In the aftermath of this event, he stopped both the Doctor's and the Master TARDIS in the Time Vortex and, appearing as a huge translucent female face, asked the Doctor whether he wanted Kronos to destroy him. The Doctor demured, Kronos let him free and the Master vanished into his TARDIS. (DW: The Time Monster)
Behind the Scenes
The name Kronos refers back to Greek mythology; Kronos was the father of the Olympian Gods. Kronos is similar to the word chronos meaning time.