What controversial dark fantasy anime series follows Keyaru, a healer who, after being exploited and tortured, gains the ability to reset time and seeks revenge on those who wronged him?

The anime series that fits this description is "Redo of Healer (Review)," known in Japan as "Kaifuku Jutsushi no Yarinaoshi." This dark fantasy follows Keyaru, a character initially introduced as the "Healing Hero" who possesses potent restorative abilities. However, instead of being celebrated, Keyaru is exploited, drugged, and subjected to horrific abuse and torture by his supposed allies, the other heroes of the kingdom. This profound suffering fuels his desire for retribution.

Keyaru's unique healing magic eventually allows him to not only recover from his torment but also to "heal" the world itself by resetting time by four years, retaining all his memories and skills from the previous timeline. Armed with this knowledge and newfound power, he embarks on a brutal quest for revenge against those who wronged him, meticulously planning their downfall. The series delves into themes of trauma, power, and justice, exploring the moral ambiguities of an anti-hero seeking retribution in an unforgiving world.

"Redo of Healer" gained significant notoriety for its extremely graphic and explicit content, encompassing both violence and sexual themes, which led to widespread discussion and controversy upon its release. Critics and viewers have debated whether the explicit scenes serve the plot's exploration of revenge and abuse or are simply gratuitous. Despite, or perhaps because of, its provocative nature, the series has garnered a dedicated audience interested in its challenging narrative and its unflinching portrayal of an abused protagonist's dark path to vengeance.