Crystal of All Power

The Crystal of All Power was created by the Masters of Karn. It led to their destruction. Before that happened, it was broken into seven segments and scattered over the planet. An alternate Fourth Doctor found the pieces, which were then taken by the Daleks.

However, he was able to alter the molecular structure of one of the segments. This caused their Ultimate Weapon to explode, presumably destroying the Crystal as well. (NOTVALID: Seven Keys to Doomsday)