Somewhere Beyond The Sea by TJ Klune

The sequel to The House in the Cerulean Sea!
 
A magical house. A secret past. A summons that could change everything.

Arthur Parnassus has built a good life on the ashes of a bad one. He’s headmaster at an orphanage for magical children, on a peculiar island, assisted by love-of-his-life Linus Baker.

And together, they’ll do anything to protect their extraordinary and powerful charges. However, when Arthur is forced to make a public statement about his dark past, he finds himself fighting for those under his care. It’s also a fight for the better future that all magical people deserve.

Then when a new magical child joins their island home, Arthur knows they’ve reached breaking point. The child finds power in calling himself a monster, a name Arthur has tried so hard to banish to protect his children. Challenged from within and without, their volatile family might grow stronger.