Monogatari S00E20

When Yotsugi tells Koyomi that he is indeed becoming a vampire, Koyomi reasons that using his vampiric powers must have come with a price. Yozuru tells him that while there is no way to undo the progress of his transformation, there is a way he can stop it from progressing further: he must no longer use his powers as a vampire. However, this turns out to be quite a challenge when he learns his sisters and Suraga have been kidnapped.