Kosuke Uchiha gained this ability when he accidentally discovered a door to a world full of demons deep within the Sound Village. Here Kosuke absorbed much power, but eventually past out. Before he came to he had been found by his father, Sasuke Uchiha and his brother, Gisuke Uchiha. When he came to he grabbed his father and brother's arm and accidentally infused them with his dark energy temporarily, transforming them. They were similiar to the Stage 2 of Sasuke's cursed seal. Sasuke could feel immense power flowing through him even after he changed back. This is the reason he keeps Kosuke with him or Gisuke whenever they do a mission. Apparently this ability can also repair the damage done by the Mangekyo Sharingan as evidenced when he healed his brother's eyes during the transformation, and his eyes were fine after it wore off.
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