Shi ni Modotta Mob wa Last Boss no Saiai deshita

Kreis, the son of a powerful marquess, is actually a modern-day Japanese man who's been reincarnated into the world of a BL novel...that he wrote! Unfortunately, his memories don't return until after he's been branded a villain, executed as a lackey of the final boss, and sent back in time. Now that he has a second chance at life, Kreis swears revenge on the family that mistreated him and vengeance for the story's stunningly beautiful final boss: King Siegfried. For some reason, Kreis is overwhelmed with a desire to protect him, no matter the cost. Kreis knows that all the main characters of the story, including his younger brother—the story's original protagonist—will soon gather for his birthday party. With Siegfried's support, will Kreis finally break free from the story's predetermined path?

(Source: J-Novel Club)