One advice to anyone who wants to start this series: start from Volume 1, doesn't matter if you watched the anime, it's better if you read the novel from the begining so you don't lose the insane amount of foreshadowing this series has. And please do me a favour and don't drop this novel unless you've read Volume 3, why you ask? Well, it's because Saijaku Muhai no Bahamut is the perfect definition of the "it gets better" mantra.
The first 2 volumes are very generic, with Lux being extremely overpowered and unrivaled. But then, the third volume shows a deeper plot, the characters start showing their true nature beyond that generic outside cover, and the battles go up a notch.
It is, however, the events between volumes 6 and 14 that truly show why I rate this series as a 10. Much more complex and compelling antagonists such as Singlen appear, Lux is constantly thrown against the ropes due to the new levels of threat he's fighting against, the girls are developed as individuals while their relationship with Lux moves forward, all the while the interesting mystery of the world's nature exists on the background.
And then volume 15 manages to top everything before it, giving us one of the most amazing battles I've ever seen on a light novel, explaining us who the main antagonist Fugil is, and setting a way darker tone that lets the political drama aspect the series had been scratching up to that point fully shine, with all the next volumes following from everything promised in here.
So despite starting as an apparently generic and unremarkable novel, the true nature of this series is amazing political drama with epic battle scenes and antagonists that totally steal the show and your heart, all while giving us a harem with actually developing romantic bonds not only from the girls' side, but from Lux's as well. Edit: And to tell you about who ends with, the answer is True Harem Ending, he marries all.
So I invite you to enjoy this beautiful, fully translated light novel about the story of the birth of a true hero.