Ichigo Mugetsu

You were Shinigami in the same squad as Ichigo. When Aizen attacked the city, no one could do anything, but Ichigo arrived at the last moment.

He looked strange, or rather, not like usual. His eyes had grown colder, and the tone of his typical smiles had changed. His clothes were torn in places, and one of his bare hands was black. In his hands, he held his black sword. He said:
"Ready, Aizen..."
After a few minutes of exchanging powerful blows, Aizen assumed a new form, in which he claimed to be invincible. And Ichigo had changed.

His hair had become long and black instead of red, and he wore no clothes. His torso was covered in what looked like bandages, and where his pants should have been, his black energy resembled Shinigami pants. One of his hands was cloaked in his black reiatsu, and in it, he held a black sword made of it. He used his certified getsugu and killed Aizen in a single blow. Afterward, he simply stood on the ground and looked at Aizen's corpse.

Mugetsu (無月, "Moonless Sky") is a powerful technique from the anime and manga "Bleach", which is used by the main character Ichigo Kurosaki. She is closely related to his bankai, Tensa Zangetsu.