There are no spoilers for 4.13 - I wrote this before the episode aired and only took information from the trailer at the end of 4.12. So it’s very AU!
Title: Draw This Line
Rating: R just to be safe [for violence]
Warnings: Torture, character death (not Arthur or Merlin), battle-type violence, restraint (shackles).
Pairing: Arthur/Merlin
Length: ~5000
Summary: One person in a battle can make all the difference. Merlin learns that the hard way when Helios joins Agravaine and he’s cornered in the caves. He’s taken to Camelot, where Arthur and the knights lay in chains, waiting as Morgana eagerly anticipates finding out exactly who the mysterious Emrys is.
Story Link Here