Rewriting History Starting from Summer Time Rendering

Being reborn gave him new hope, but destruction was already closing in on this new world. Forced to leave his homeland, he takes its last hope with him: a holy sword capable of opposing an overwhelmingly powerful enemy. Thus begins a journey across time to rewrite history. (P.S. I have seven older novels with over a million characters, so quality is guaranteed. I'm terrible at titles and blurbs... For details, please read the story. Also, before the title was changed, it was called The Journey of a Fugitive Knight. It begins with the 500-years-ago storyline of What Do You Do at the Start of the Apocalypse? Soon after, the story moves into a fused world of Lycoris Recoil and Summer Time Rendering.)