This is a one-handed longsword created by Lisbeth for the use by Kirito, who weilds it alongside the ELucidator when he uses his ‘Dual Blades’ skill.
In the anime, Dark Repulser is aqua-colored. Its grip is a darker color than the rest of the blade and is completely straight. It leads up to a crossguard that is exactly perpendicular to the grip and is designed to look like a pair of conjoined dagger blades. At the center of the guard is a large aquamarine gem.
Kirito, also known as Kirigaya Kazuto, is the main protagonist in the Sword Art Online Series. Due to his tendency to wear black clothing, he became known as the ‘Black Swordsman’.