Crystal of Kronos
At the moment of Kronos's birth, the Guardians of Time imprisoned it in the Crystal, which was created by the Key to Time, and then hurled into the Time Vortex, to land wherever it was carried. The crystal was hidden throughout time on many worlds until it arrived on Dæmos. The Dæmons brought it to Earth, where they gave it to the people of Atlantis who used it to help their civilisation flourish. (PROSE: The Quantum Archangel) King Dalios later forbade use of the Crystal, knowing that it would ultimately doom his people.
In the 1970s, the Master acquired a fragment of the Crystal, and used it to manifest Kronos in the hopes that he could bind Kronos to his will. The Master failed and, returned to the later days of Atlantis, Kronos destroyed the city. The Crystal itself was destroyed when the Doctor's TARDIS time rammed the Master's TARDIS, freeing Kronos. (TV: The Time Monster)