The lightsaber was a Sith lightsaber, legend says anyone who attempts to wield it will be driven mad.
During the Clone Wars, the Sith Lord Darth Sidious tasked General Grievous with retrieving the saber for him, only for him to be bested by Maul, Sidious's former apprentice. When Maul returned to Coruscant, the lightsaber broke upon activation. Displeased, Sidious tossed his former apprentice out of the window while he kept the broken saber.
The saber remained in Darth Vader's castle by 35 ABY, when Vaneé hoping to fulfill the Prophecy of Mustafar, retold the story of what happened before while Dean repaired the saber.