Tokijin is one of Sesshoumaru's two swords. It was forged for him by Kaijinbo (a former apprentice of Totosai) and is an immensely evil sword. When it was first forged the evil within it took control of Kaijinbo and forced him to fight Inuyasha. Tokijin is Sesshoumaru's preferred choice of weapon and uses it more often than Tenseiga. Tokijin was made from the fang of a demon slain by Inu Yasha when he was in his demonic form. The hate of dieing by Inu Yasha is what took over Kaijinbo when it was first forged. Sesshoumaru, being more powerful, can control this hate and does not let it overcome him.
Later in the series, during a battle with Moryomaru (armor created for Akago), Sesshoumaru breaks the Tokijin in an attempt to break through Moryomaru's armor. He never attempts to fix it in anyway, even cursing it as he tosses it aside, never to be used again.