Ninjago

When Morro possessed Lloyd and took him hostage, his subordinates tended to Lloyd's body. When Morro no longer needed his physical body, they couldn't return him to the ninja monastery, so they erased his memory. Too much... When Lloyd awoke, he remembered only his name and nothing else.

"It seems your spell worked too well..."

"Morro muttered sharply, looking at Lloyd.

For several weeks, Morro instilled in Lloyd the idea that he was a villain and taught him to use his powers again, but in a more brutal manner. Lloyd's character changed, becoming ruthless and cruel, and siding with Morro.