Kronos is the most fearsome of the Chronovores, creatures which live outside space-time, a place that is no place. They are time-eaters, able to swallow an entire life. Kronos was originally drawn into time by the priests of Atlantis, using a trident-shaped quartz crystal as its center, into which they were able to capture Kronos. Kronos spawned the Greek legend of the Titan who ate his children, one of whom was Poseidon, god of Atlantis. The Atlanteans called him the Ruler of Time, the Lord of Time, the Mighty One, and the Destroyer.
There is an implication that the crystal is anchored to the time period of Atlantis, but other versions of it exist throughout time. The Master is able to gain control of the crystal in the twentieth century, and he uses it as part of an experiment to transmit matter instantenously by moving it through the interstices between "atoms" of time. This apparently stimulated activity in the crystal where Kronos was trapped, causing it to glow. Eventually the Master was able to bring the High Priest of Atlantis forward in time, and using the high priest's seal, he summoned Kronos successfully and was able to keep it under control - or so the Master thought.
This manifestation of Kronos appears as a tall, regal white creature with birdlike wings. It flies through the air and smothers people with its wings, which immediately transports them outside the space-time continuum.
The Master eventually decides he must travel to Atlantis in order to recover the true crystal of Kronos, which is kept in a special room and guarded by the Minotaur. Once the Master gets control of the crystal, he frees Kronos again. Kronos goes on a rampage, causing destruction all over Atlantis. In order to destroy the crystal, the Doctor and Jo create a time-ram by materializing the Doctor's TARDIS in the same space as the Master's TARDIS exists. Ordinarily this would have destroyed both TARDISes and their occupants, but as it also destroyed the quartz crystal and freed Kronos, Kronos was able to bring the TARDISes to the boundary between human reality and that of the Chronovores.
Here Kronos appeared to them again, this time as a female with silver skin and gold-lidded eyes. Kronos revealed to them that it was beyond good and evil in human terms; it was a destroyer, but also a healer and a creator. For releasing Kronos from the crystal, it owes the Doctor a debt of gratitude. It grants his wish to be sent home, but intends to torment the Master in return for what he has done. The Doctor pleads for the Master's life and freedom; Kronos grants it, but then does not prevent the Master from escaping in his own TARDIS.
