Need help finding a Merlin fic

[multi_fandom18]

Hey, hope you're doing well!

It's a merthur canon au magic reveal fic. Arthur finds out about Merlin's magic and doesn't take it well and Merlin is feeling guilty so he wears cold iron handcuffs to block his magic and writes a letter to Arthur vowing that the magic was only ever meant for Arthur so he won't use it till Arthur tells him to. But

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[serafina20]

It might maybe be To You I Pledge which has handcuffs but I don't remember if there's a letter.