@junaria - in his previous life, MC was a crafter in a magitech civilization, named Gaias. But what he liked most, was fighting. He lived for a thousand years, hit level cap, crafted the most powerful weapons, had a reputation as an overpowered sage or maybe even a god, but still wasn't overpowered enough. He wanted to go to space, and kill everybody there too.
So he cast reincarnation magic, hoping to be born with talent for fighting instead of crafting. He succeeded and is born with a talent for close combat, which is very much to his liking. And so it was time to amass power and equipment once again, gather a party of like-minded individuals, and conquer space.
Well, while he was away, the world has degraded to fantasy middle ages level. So he has a hard time finding good equipment and capable individuals, and so he decided to just raise some of his own - he just grabbed a couple of teenage girls who were his classmates, and a dragon that he knew once before. He also spread his knowledge (well, the easy parts) among the populace freely.
Things didn't go according to his plan, because while he was away, demons have multiplied, and their conflict with humanity is entering "hot" phase. So MC has to defeat the demons first. Fortunately, MC is the foremost expert on slaying demons (and everything else), who nearly drove them to extinction the last time.
His current plan is to find the oldest and most powerful demon around and kill him. Which he plans to do with the magitech pyramid they found this chapter.
@LegitRikk - in Mathias' past life, reincarnation magic, and any other magic that messes with souls, was considered heretical. Which is why Mathias prefers to keep that part of the story a secret.