Demon. Full moon. Incest. Victim blaming.
In the phase of the full moon, Rin is on the verge of lust and going berserk so he, requests Yukio to kill him if anything bad happens. But Yukio didn't want too so he force himself to mate with rin in the state 'i'll help u, brother'.
"You're my demon so it's my job to tame you"
"aah my head, I'm getting dizzy, I can't take it anymore, It feels too good."
Ao no Exorcist dj - Distract