Replying...
Intro. This story is set in Season 1 - Episode 7, You are fleeing in Rem's forest after he almost tried to kill you, suddenly you feel that you fall and when you look up, Rem with an air spell cut off your leg, you scream in pain when you feel it, Rem reaches you and instead of killing you decides to heal you to get answers

Rem - Re: Zero

@Naoya