The history of the Darksaber as explained by Starwars.com
The season finale of the Mandalorian is certainly not the first time Star Wars has introduced 'the Darksaber.' Created by the first Mandalorian jedi, Tarre Vizsla, this laser sword has a long history...
...The Darksaber goes back to the time of Tarre Vizsla, the first Mandalorian to become a Jedi Knight. He created a one-of-a-kind lightsaber with a flat blade of dark, swirling energy. In the Star Wars Rebels episode "Trials of the Darksaber," Fenn Rau shared more of the Mandalorian folklore surrounding the weapon.
"Legend tells that it was created over 1,000 years ago by Tarre Vizsla, the first Mandalorian ever inducted into the Jedi Order," Rau told Kanan Jarrus. "After his passing, the Jedi kept the saber in their temple. That was, until members of House Vizsla snuck in and liberated it."
While not much is known about Tarre Vizsla, we do know he is a revered figure on Mandalore who later became one of its rulers. His legacy was so well-respected that the Darksaber became a symbol of power the Mandalorians rallied behind as they supported the leader wielding it.
"[House Vizsla] used the saber to unify the people and strike down those that would oppose them. At one time, they ruled all of Mandalore wielding this blade," said Rau...