Making the Villian haunt the narrative
A very, VERY simplified version of this story: Basically, in my story there was this great evil that was created due to a hundred year war. This evil was then 'defeated' by three heroes, with two of them sacrificing themselves and only one surviving, the main character's mentor. However, the evil wasn't really defeated but instead sealed, and only a few people know this. Later on, not only does the evil escape and become the final villain, but it's revealed that it had been working from the shadows to free itself.
So how do I make them 'haunt' the narrative, not only so that their eventual reveal doesn't feel forced or out of nowhere, but also so that they had an effect on the world and on people? Destruction and abandoned battlefields don't really work since there were already multiple wars before it. I tried to think something else up but came up with nothing