15Sauron ruled it for 2,500 years
Sauron has been around almost since the beginning of Middle-earth. He’s a Maiar, beings just one order in power below the Valar, whom they helped form the world from the songs of the ultimate creator, Eru Ilúvatar. That’s a lot of fantasy names to process, but just know that Sauron,
Gandalf, and Balrogs are roughly the same “species.”
In the First Age, Sauron fell from grace to join Morgoth in his bid to destroy the world. Like many evil sidekicks, he ran away and hid the moment things started to look bad for his boss. This was after he commanded a legion of werewolves, but that’s a long story.
After waiting a polite 1,000 years after his master’s defeat, the second Dark Lord built his tower in Mordor and hatched his own evil plans until his first defeat 2,441 years later. So he had a good run.