Weird how this is different from the webnovel. Must be material from the LN or just the mangaka setting things up to axe it.
Ill give some spoilers to those who want to know what happens next. Read at your own risk:
Originally this night was supposed to happen almost two years later. Originally his sister catches the lazy disease and is stuck in a coma. He spend years researching how to cure her. After he defeats the demon, his increase in magic power gives him the insight to finally cure her. He then discovers that magic originally existed, but it has been erased from history because the people who defeated the demons, did so with magic and they were half demons themselves. It is also revealed that he's the last descendant from them and was originally entrusted to the country's queen by a mysterious person.
The next arc deals with him going to the capital, gaining a nobility and curing everyone over there. also setting up a lengthy class to teach others to manipulate magic and cure people in other countries. He faces many backlash from other nobles.
The arc after that and current one. Is he having extracted a promise from the queen for the settlement of a magic school and development of magic weapons due to the demon treat. Goes to another country to research the fairy's forest. There he finds a diary of another Japanese. And is now into deep linguistics territory trying to decipher the fairies language that uses magic in its structure. All with the help of a local noble who he made a mage in the last arcs kingdom sponsored lessons.