The Darksaber was an ancient, black-bladed Lightsaber. According to legend, it was created by Tarre Vizsla, the first Mandalorian ever inducted into the Jedi Order. Following his passing, the weapon was kept in the Jedi Temple until members of House Vizsla stole it during the Fall of the Old Republic. The Darksaber became a symbol of leadership on Mandalore, and was passed down, generation to generation, by the ancestors of Pre Vizsla until it was taken by Maul, former Sith apprentice of Darth Sidious, and with it, leadership of Death Watch.
After the Clone Wars, Maul added the Darksaber to a collection of artifacts he maintained on Dathomir, and it was then taken up by Sabine Wren after she was possessed by the ghost of one of the Nightsisters. After being freed, Sabine recovered the Darksaber for her own use, and would eventually be instructed in its use in hopes that she could use it to unite the people of Mandalore.
Returning home to Concorde, Sabine faced her family, specifically her mother and brother, but Ursa Wren believed the Darksaber wouldn;t change anything, and so went behind her daughter's back and contacted Gar Saxon. In exchange for her daughter's safety, Ursa would deliver to Saxon the Darksaber and the two jedi accompanying her. This deal backfired when Saxon intended to wipe out Clan Wren to cover up his own crimes, and he attempted to use the Darksaber to kill Ursa, only for Sabine, wielding Ezra's lightsaber, to come to her mother's defense, leading to a ferocious duel between the Mandalorian Rebel and the Mandalorian traitor. Sabine bested Saxon but chose to spare him, and when Saxon tried to shoot Sabine in the back Ursa executed him with a blaster-bolt to his heart.
Sometime later, on the planet Mandalore, Sabine wielded it in a mission to free her father, Alrich Wren, but almsot fell to a trap laid by Saxon's brother and successor, Tiber Saxon. Thankfully, the trap was foiled by the timely arrival of Bo-Katan Kryze and two of her loyalists. Sabine offered the Darksaber to Bo-Katan but the Mandalorian veteran declined, citing she was not her sister and that she'd had her chance to rule but had failed.
After the mission to destroy Sabine's weapon, Sabine again offered the Darksaber to Bo-Katan, stating the veteran had the wisdom and guidance of a true leader, and believed she (Sabine) had found the Darksaber because she was meant to pass it on to Bo-Katan. After several clans, including Wren, swore loyalty to Clan Kryze, Bo-Katan accepted the Darksaber and became the true and rightful leader of Mandalore.