Limerence (слэш)

'The potion in question was, as far as I can tell, of suitable strength to do its job. Merlin's power would have offered him no defence. There is, of course, one universal antidote against any magics that meddle in matters of the heart...'
'Which is?' Arthur demanded, raising his eyebrows.
Gaius sighed, casting a tired, apologetic look in Merlin's direction. 'If the target of a love spell is already truly, deeply devoted to someone – if they love another without question or pause – then no spell can take hold.'
'You're saying that it didn't work because Merlin already loves someone?' The words felt thin between his lips, straining to get past the sudden, leaden feeling in his chest. It felt like someone had plunged their hand inside him and caught his heart in the cage of their fist, and Arthur was left breathless as he tried to speak around the awful sensation.
******
When a love potion fails to affect Merlin because his heart has already been claimed, Arthur is left wondering who has earned Merlin's devotion