Guts. Not much is known about this man. Mages have investigated into this character, only to discover old transcripts that mention king Arthur. Many have tried to summon him. All have failed.
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)