Happily Ever After || l.s. a/b/o

Harry Styles, son of the alpha of the Styles Pack, is an omega.
Louis Tomlinson, son of the alpha of the Tomlinson Pack, is an alpha.

Their families get them engaged when they were only four (Harry) and six (Louis) as an alliance between the packs. It was decided that the mating will take place six months after Louis' eighteenth birthday as it will be time for him to take over the pack.

What happens when it's time for the mating and for Louis to take over the Tomlinson pack as he is eighteen now?