Tsukishima was the original founder of the XCution group, and formulated the plan to pass their powers on to a Shinigami Representative. However, as soon as this process began, he had a change of heart and used his abilities to kill the Shinigami and those who had been relieved of their powers. The reason it's still unknown, maybe it was his plan all along.
Tsukishima is a primary villain in the "Lost Agent Arc". He manipulates Ichigo friends and family members, making them unvoluntary allies, torturing the mind of the hero, before physically confronting him. Ichigo was able to stand his ground, even cutting of one of Tsukishima arms. The villain used Orihime's powers to heal up and keep up the fighting. Later, as Ginjou is revealed to be the true mastermind, the Gotei 13 joins the battle against the XCution. Tsukishima ends up fighting Byakuya. The long and violent battle, the Captain manages to defeat the the villain, piercing his chest and leaving him on the ground.
Tsukishima is a fullbringer. His fullbring manifests itself in his bookmark, which he can transform into a katana that has a bookmark-shaped tsuba. Ginjō remarks that the sword posses an high attack capability that can literally cut through anything. It's named "Book of the End". The sword real powers allows Tsukishima to "insert" himself inside the history of everything it cut: he can slice a man from behind, leave no physical wounds on the outside, but placing himself inside the memories of his victim in any given role. He can become an allied, an enemy, a dear friend, even a family member. The effects can be nullified with a second strike on the front of the victim. This ability works even with places and inanimate objects: he can scracth the surface of a building and make it so he was there before, placing traps or tricks that weren't actually there before.