The crystal of Kronos in Atlantis. (TV: The Time Monster)
The Crystal of Kronos was a trident-shaped crystal which contained the most fearful and powerful Chronovore, Kronos, (TV: The Time Monster) within a multidimensional prison.
At the moment of Kronos's birth, the Guardians of Time used the Key to Time to imprison it within the Crystal, afterwards hurling it into the Time Vortex to land wherever it was carried. The crystal was hidden on millions of worlds since the dawn of time, including Skaro, until it arrived on Dæmos. The Dæmons then brought it to Earth, where they gave it to the people of Atlantis 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)