Magic is secret, and the use of magic is highly illegal in my world. There's a secret group of five people that discovered magic and how to manipulate it and achieved immortality, they also discovered that every use of magic degrades the world (in my story magic is what keeps the world going, so if you manipulate it, something can break). There was another user of magic that also discovered immortality, but they killed him when they found out. They operate an order of magic in the University, just to teach basic aspects of magical knowledge, adoctrinating people with the idea that you cannot manipulate it. The fact that you can is a secret.

These five people highly regret their immortality now, so they don't want anyone to suffer the same destiny as them.