Merlin gets kidnapped by an unknown person and when he returns, he's different. Gaius finds out what causes it. The Fomorroh. Arthur and Merlin go on a quest to kill the mother beast while it's tranquilized, so they can protect Camelot from Morgause and Cenred. Gwen and Morgana are also a big part of the story and I write in their perspective too. (I have put it on AO3 as well, I am currently editing out spelling/grammatical errors there, because it's easier than on this platform)