The Butterfly of Black Death - S1.E6

Kazuki finds Koushaku in his great-great grandfather's laboratory and attempts to destroy the Homunculus matter. However, due to the interference of Koushaku's younger brother, Koushaku is able to have the Homunculus matter attach to him and is turned into a Homunculus. He then begins to eat everyone in the vicinity until he is killed by Kazuki, who passes out from exhaustion before he is able to deliver the cure to Tokiko. Kazuki awakens to find that Tokiko has been cured and that his buso renkin shall be called Sunlight Heart.