Caged Pheonix (Harry Potter Fanfic)

Sirius knows Regulus is dead, yet finds Orion's belief that this isn't true just as contagious as what his grandfather said, that Regulus challenged a magical being without finishing what he started. The contagious thought leads to a different boy-who-lived, and... At the end of the day, Regulus needs to finish what he started, and for that to happen, Sirius realizes eventually, he needs to find Regulus, because that is what will keep his godson safe, right? Disclaimer - I don't own Harry Potter. This is written for ONC 2025, prompt 13. "When you challenge a magical being, the rules are simple: you must finish what you started, no matter the cost."