Dark Repulser is a one-handed magic longsword which Kirito used as his secondary weapon toward the end of the Aincrad arc of Sword Art Online. It reappears as his secondary weapon in Kingdom Hearts vs. Sword Art Online, beginning later during the Keyblade arc.
The sword was crafted by Lisbeth from a rare metal that she and Kirito found in the western mountains on floor 55 of Aincrad, and is the best sword she's forged in SAO.