The Mc wakes up to a whole other world. The time period is back in the Medieval times, but with a twist, there are stuff only thought to exist in a fantasy world.
Dragons, demons, gods, goblins, etc. exist in this new world.
We can't forget the most important thing in a fantasy world, MAGIC!
The Mc starts their journey of self-discovery and their rise to power that can rival that of gods'.
Nearing the middle end of the book, they become the god of magic, surpassing the past god.
After becoming a god, they discover a shocking truth.
This isn't a whole new world, this is Earth, but just way back then.
As the years advanced, the mana produced by the world slowly decreased, leading to magic and anything mana-related disappearing.
That is why there is nothing magic-related or mana existing now.
As a god, the Mc is made of mana, so eventually, even they will disappear. No matter what they do, it is all useless in the end.
So, the Mc made a plan. The plan was simple, but very complex at the same time.
Step 1, locate the future(?) or past(?) them.
Step 2, bring them(future or past) before They(Mc), discover magic and mana.
Step 3, just hope they(future or past) discover the truth earlier before they become a god and keep mana alive. That's it.
The reason why They(Mc), wants to keep mana alive? It is to save the world from ending. Mana is what keeps the world alive. Without it, the world will end, one way or another.