This takes place a while after the Mephyland incident. Rin notices a strange black tattoo that glows a faint blue when he's near demons. Yukio finds out and tells him the meaning of the 'totally cool' tattoo.

"When you called and said you needed me in Assiah, I didn't think this was what you had in mind." "Trust me, Amaimon, it very much wasn't, I had no idea he was even capable of these heats." Rin's issues happen to include more than being in a tug of war between the two halves of himself. Now, they include demonic hormones too. If his body doesn't kill him before the week is up, he might settle for killing Mephisto. At least Amaimon is having fun. (As always, characters are aged up appropriately, and the demon kings are NOT related).