Well, I'm not sure about a specific titled 'Regulus Leaves with Sirius' fanfiction off the top of my head. But if you search on platforms like Fanfiction.net, you might come across some great works. Usually, in such fanfics, the authors explore the complex bond between the two characters. They might depict Regulus leaving due to his loyalty to the Black family's dark secrets, while Sirius is on the opposite side, and this creates a lot of drama and heartache in the story.
One popular 'Regulus Leaves with Sirius' fanfiction is 'The Departure's Echo'. It beautifully portrays their relationship and the emotional turmoil of Regulus leaving. Another good one might be 'Farewell in the Shadows' which delves deep into the reasons behind Regulus's decision and how Sirius reacts to it. You can often find these on major fanfiction platforms like Archive of Our Own.
There are quite a few fanfictions that could fall under this theme. Some fanfic writers like to focus on the magical world's politics and how it affects their relationship. For example, a story could show that Regulus leaves to protect Sirius from a certain danger that he has uncovered in the Death Eater ranks. This would lead to a very angsty and emotional journey for both characters, with Sirius feeling betrayed at first and then slowly realizing the true motives behind Regulus's actions. You can try using specific tags on fanfiction platforms to narrow down your search for such stories.
Chapter 474, ", Showing You the Corpse", is the clearest early answer because it turns Fury Flame Battle's situation into a concrete plot event. Early on, To kill Titus in a one-on-one duel was to usurp his position; to lead.... By the end, He wanted the head of Titus, not Muru..
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