I wonder if I'll ever understand the plot...
Plot in a nutshell:
Ancient demon lord woke up from the slumber. But the world has changed - it is now about students with superpowers ("sacred swordmen") battling monsters of the week ("Voids").
Fortunately, demon lord woke up as a cute shota, and had no problems infiltrating the school and even getting into a dorm with big-tittied onee-chans.
Foul play is suspected:
- people are aiding Voids. It doesn't look like they invented a cool name for themselves, which makes explanation harder.
- people are transforming monsters into Voids (new!)
- information about ancient demon lord and majority of that period is not available.
- magic is almost unknown, even while magitech has reached unheard-of heights.
Conflict of interests is outlined:
- Ancient demon lord served a dark (but pretty) goddess. His memories of their relationship are hazy.
- Ancient demon lord waits for her revival and wants to throw the world to her feet.
- people who are aiding Voids are also serving a goddess. Probably the same one.
- they serve her by aiding Voids and attempting to resurrect her as a void.
- Ancient demon lord doesn't want goddess to be resurrected as a void. Because it's a harem anime, and goddess must be resurrected as a waifu.
Note that this manga, like the author's previous work, follows a rhythm: each arc starts with some relaxing slice of life, that steadily grows into a confrontation, eventually reaching a climatic battle and its resolution. The plot is mostly seen in "grows into a confrontation" stage, while the SoL and the battle stages are mostly fanservice.