The Unbreakable Vow
“And will you, by the best of your ability, protect her from harm?”
“I will,” said Sherlock.
A sencond tongue of flame shot from the wand and interlinked with the first, making a fine, glowing chain.
Do you recall this happening in S3? No? Well neither do I, so why does everyone act as if Sherlock had made an Unbreakable Vow to protect Mary? I can’t be the only one being reminded of this?