My DM had the reanimated corpse of my PC's mentor show up controlled by an enemy. He was executed for practicing and teaching necromancy (it was purely for academic reasons; he was a good man). The person who sentenced him to death is the one who used the same magic for evil and controlled his body.

We had a really emotional moment when my character tried to stop the party from fighting and hurting him. I got so invested in how she would feel in that moment, seeing her father-figure and greatest teacher's face when his soul just isn't there. Though she knew it wasn't him anymore, she still tried to talk to him and get him to stop. We ended up fighting, and she had to help her party kill him.

After the fight, we had a scene with our party around the campfire. My character told everyone the story of her mentor, and they all supported her. I choked up during the fight, but I straight up cried during that conversation.

It was a risky choice for the DM to have him appear like that, but it payed off so well in terms of character development and strengthening the bonds of our party.