The Lightsaber Crystal of Darth Plagueis originated as a Korriban Sigil Crystal. Sigil crystals are known for creating lighsaber blades that radiate intense heat to the point of igniting flammable material within six inches of the blade. Through intense meditation prolonged exposure to Darth Plagueis, this crystal radiates the Dark Side of the Force.
When Scorpius discovered the Sith Holocron, this crystal was also found hidden inside the holocron's first teir. He returned both items to the Clan Summit, where they were made official, and Scorpius given credit for finding them. Today, the crystal is kept locked away in vaults deep within the Dark Tower, where no one, not even the Consul, can use it.