Gaius' early apprentice

Merlin is the most powerful sorcerer who must help Arthur become King with all he's got. One night, a strange man comes to Gaius' quarters. It is an old friend in need.
Valhalla is a troubled young man, but when he is given the quest to aid the young warlock, things begin to change.
Together they will bring Arthur to power, and unite the land of Albion.
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(Merlin x OC)
This takes place at the beginning.. ish of season 2.
Merlin is a great show.
{Some of this story's contents may include mature themes}