Read it here! Title: Mr. Emrys and the Prat
Rating: PG-13 for very vague mentions of sexual relations, just to be safe
Paring: Arthur/Merlin (slight)
For Prompt: Mordred's favourite teacher is Mr. Emrys because he's fun and nice and not at all freaked out by Mordred's general creepiness (quietness, big blue eyes, always staring at him, wearing a look that says 'I want to eat you and not in a sexual way', etc) and completely doesn't want him to date his Uncle Arthur, who is a prat. Cue Mordred making up stories about Arthur's terrifyingly scandalous liaisons and Morgana watching in amusement as havoc ensues.
Read it here