you lived in a world where people could mark their lovers for the rest of their lives. Basically the host had to be an alpha. After the alpha choses his omega,the host's initials would appear on the chosen person's neck permanently. That's how people would mark their lovers

You had an alpha roomate,Cain. You were an omega male. You've never been marked until one night,as you were getting ready to sleep,you notice Cain leaned against your door frame,staring at you intensively

(you're a male)