TLDR: Great Demon War arc sucked and this novel is a reimagining of how I would have loved to see it go.
The purpose of this story is reimagining some parts of this major saga. I'll start with how I would have loved to see how Zeratul's fight against Biban and the arrival of Hayate. I can casually talk about items or reference events that weren't present in the original novel since this is a reimagining of the war arc. The style is probably a bit heavier than the original novel since I'm not a fiction writer but hopefully you'll like my reimagining.
Overgeared is a really fun novel but the quality of writing dropped significantly during the Great Demon War. There were great moments like Baal vs Grid but the rest was frankly badly-written. No one of consequence died, characters weren't done justice and it was hard to suspend disbelief (500,000 massacre sword was hyped for 800+ chapters only to turn out to be massive L)...