Oh, this is gorgeous! Merlin looks a tiny bit speechless. I can so imagine the story behind this. Perhaps Arthur is through spurning the princesses Uther has paraded before him. In a moment of clarity, Arthur decides to act on his true feelings!
By the way, I am 'gwyllion' on AO3 and the link to your art showed up in my Google alert this morning. Nice to meet you! (Gwyllion was my sweet cat's name from 1987-2008. Although he lived to the ripe old age of 21, his beautiful name lives on!)