The Keystone

The Keystone was a plutonic crystal created by the Sea Devils as an embodiment of their power, a "formidable gem" with "infinite powers".

Nature
It had the appearance of a large, orange jewel that could glow "unexpectedly". It had many abilities, being reputedly capable of "chang[ing] the course of the world", "freez[ing] life and time", and "transporting matter". It could, for example, be used to freeze an individual in time and encase them in a stone statue, although this stasis field would fail if the stone coating was destroyed. Additionally, by combining it with underwater Sea Devil fluo-geomagnetic systems, the Chief Sea Devil intended to "flip the Earth's geomagnetic poles, south to north, north to south, longitude to latitude" in order to "create chaos, melt the ice, shift the current, change the pressure" and ultimately flood the entire Earth, destroying humanity. (TV: Legend of the Sea Devils)

History
The Keystone was created by the Sea Devils, but found by humans, becoming part of the lost treasure of the Flor de la Mar. After it sank, it was found by the "great captain" Sin Ji-Hun. Aware of the jewel's nature, Ji-Hun entrusted it to his second-in-command Lei Bao before his ship was attacked by one Chief Sea Devil's pirate crew of Sea Devils. Though Marsissus captured Ji-Hun and his ship, Lei Bao made it to land with the Keystone. He and his descendants became its custodians, also using it to entrap the Chief Sea Devil and freeze him on dry land in the form of a large stone statue.

After he was accidentally released in 1807 by Madam Ching, Marsissus managed to recover the Keystone and put his geo-magnetic plan in motion, but the Thirteenth Doctor, working together with Ji-Hun, managed to rig his systems to explode instead. (TV: Legend of the Sea Devils)