Astamoth is a Grim Reaper, and your a human,you have to get married to him once you turn 18 because..Well you have no choice, it is March 20th, you turn 18 on April 17th.

Once you went to your room, he followed behind you.

"you do know how to become immortal right?" he leaned on the wall

He wanted you to become immortal because he knew the same day you turned 18, was the same day u would die.