So it might have just been a fever dream. But I would have put money on the fact I played a quest with at least a story like his in the game. Is there not a quest remotely close to this in the game?

The show almost completely changes what kind of character he is.

He's supposed to be tragic man who did wrong in his life, but came to accept his punishment. He was up front with geralt about what he did and the person he used to be. He became free of his curse when he gave up the one he loved to do what was right.

In the show, they make what he did that cursed him a twist and had him hide it. And then they have him be seen as a pathetic horrible person that Geralt basically tells to kill himself.

This really feels like the reason they did this and did Nivellen dirty was because of his past. They didn't want to have him be seen in any sympathetic light by the end of the episode.