The prank + legilimens
I remember not a lot of this fic, though I know it is DEFINITELY a Jegulus story.
What I DO remember is that it’s post prank and Remus and Regulus become friends, and way later down the line: spoilers ig, it’s revealed that Snape used legilimins (or however you spell it) on Sirus to find out about the Whomping Willow and Sirius as all like: “yeah, it IS my fault that Snape found out bc I should’ve had better control of my occlumency shields” and Remus is all like “baby….. no! I thought you did it maliciously!”