Introduction & AN

Regulus Black has been doing fine without James since that night three years ago. Alright, so maybe that isn't entirely true but that didn't mean he was going to let him back in again. Regulus doesn't make the same mistake twice, and trusting James Potter with his heart may have been his biggest mistake yet.

or:
Regulus, Evan and Barty have been hunting Horcruxes together and when James and Sirius find out, they want to help out to defeat Voldemort.