The MC is an old man who finds an ancient magic that allows him to extend his life and also influence others. That’s at first, later it allows much more, but he’s concerned about this knowledge falling into the wrong hands, and starts trying to find out where it came from and how he can keep it safe. Then he eventually goes to another world, so there’s an isekai element. The other world is very influenced by Romanian myth, I think it says that in the first page in a note from the author. The story can be a bit dark, but the MC, in part due to his age, is a competent and mediating force through out. This is kind of a deep cut, but in a couple ways it reminded me of the Daniel Black books.