Nekomonogatari (Kuro) 1x4

After Koyomi spends several days begging for Shinobu's help, she entrusts him with the demon sword (妖刀) "Heartspan" (心渡), able to kill Oddities without harming the humans they possess. Koyomi manages to lure Tsubasa to Oshino's place by sending her an e-mail claiming he is under attack by a vampire, but instead of comforting her, he berates Tsubasa for allowing herself to be possessed, prompting her to attack him. After Tsubasa slices Koyomi's body in half, it is revealed that it was all part of his plan to have herself cut with the sword, which was concealed in his body. However, Tsubasa refuses to let the Cursed Cat leave her body, and Shinobu appears to restore Koyomi's body and to bring Tsubasa back to normal. The next day, Tsubasa has returned to her normal life with no memories of what happened.