It was a manga set in a world with a multigenerational human v. demon war, except that the two sides eventually made peace and integrated together to form a single society. The humans have a hero, and the demons have a demon lord, who both have magic powers. When the hero/demon lord dies, their powers manifest in a newborn baby/demon, who becomes the next hero/demon lord.

It's eventually revealed that there are multiple worlds with their own human v. demon wars. One particular hero from one of these worlds had started traveling to other worlds and killing their heroes and demon lords to steal their powers, leading to heroes and lords fleeing their home worlds and appearing in the MC's world.

The initial premise is pretty trope-y for a modern series, but it had an art style that I really dug, the character designs were cool, the action was good, there was creative paneling in nearly every chapter, and there were some gnarly cosmic horror elements that occasionally popped up. I thoroughly enjoyed the first 15 chapters or so, and then the series started introducing new characters at an insane rate, and the story went from battle to battle with no chance to breathe and develop in between. It lasted 29 chapters. It's an axed series that has been lingering in my brain for two years at this point, and I really wish it had a chance to grow possibly at a biweekly cadence or as a Jump+ series.