Welcome!
There may be magic? I’m not sure rn. All I really know is know is that the deatheaters still exist, and that’s a big reason why Regulus gets a bodyguard. He is to be the youngest deatheater in their history, which to the Blacks is the greatest achievement to put next to their names. So Regulus is to be protected at all costs.
First Regulus’ bodyguard was Barty, he was deranged, hated the Ministry and would do anything to be a deatheater. But when new people came into the picture, things got complicated. Rugulus and his bodyguard were not to have friends, or be friends. So Barty was relieved of that duty.
Just before “friends” were introduced, Sirius left. He ran away to his friend’s house. Regulus didn’t know or care for this friend, they had taken his brother from him. Put all of this pressure on him in place of his brother.
But Sirius knew a lot of things, all of which he told this friend, friends. He told them about the abuse, the deatheaters, the brother, the bodygaurds. The bodyguards!
Sometimes Remus was sure James and Sirius were going to kill themselves because ‘why not?’ Sending James as a bodyguard in the Black estate? Absolutely absurd, but quite fitting. And maybe, just maybe, it could work.
They’d have a spy and Sirius could insure his brother is safe, maybe even save him from the fate that lay in story with the deatheaters. But how to get James in? They wouldn’t let just anyone be security, especially for a job like this.
Polyjuice? Jame and Sirius thought the idea was hilarious, and smart. Jame could take Polyjuice to look like any deatheater he needed to, and he could fake a personality if he knew what to do. Now they just needed someone to be.
(Insert random unknown deatheater idk-)