Magic should be like rumors, often whispered about but rarely verified.
This is the way of magic in Nehwon. Ask the average punter on any Lankhmar street if magic exists and they’ll say yes. Ask them to describe their experiences with magic and what you’ll get are non-verifiable accounts of blessings, curses, talismanic protections, portents and signs, and general good and bad luck–basically superstition-level magic that can not be confirmed.
Everyone knows magic exists, but it would be the rare person that could relate a true occurrence that could persuade someone skeptically-minded. Yet, magic does exist.
Magic at the superstition-level does exist. Blessings, curses, talismans, etc. As readers, we know they are all real enough in the stories. However within the stories the magic can not be seen, heard, or touched. Since there is no proof, it can be argued one way or the other, and as such its existence and nature is a mystery.
Of course we know vaster magic exists as well. But it is typically beyond the experiences of your average Lankhmart or Nehwonian resident. The greater magics are usually reserved for the presence of heroes. And even at that, heroes are not always necessarily aware.
When the heroes are directly up against great magic, indisputable and full in their senses, it is a climatic and unique experience. It has great impact upon the heroes. They have all the emotions that magic should impose upon its victims–awe, fear, and dread–all that one should feel from an encounter with something unknown and alien to their daily experiences.
Magic, in those rare moments when revealed in its full and unrestrained glory, should have a lasting effect on all–the heroes of the stories, the reader, and hopefully the characters and players of our games.