This fantasy anime follows a young man named Riddle who, after losing everything to a mysterious organization, gains the ability to reverse time after a thousand years and uses his accumulated knowledge to change the past. What is the title of this series?

The series you're thinking of is titled "The Regression of Great Sage Riddle." This fantasy anime accurately portrays the journey of its protagonist, Riddle, who endures unimaginable loss when a mysterious entity known as the Box of Malice devastates his world and claims the lives of all his companions. Plunged into despair, Riddle dedicates a millennium to a singular, desperate pursuit: mastering the ability to reverse time itself.

Upon successfully rewinding a thousand years, Riddle finds himself back in his younger body, armed with a lifetime's worth of knowledge and experience. This unique premise allows him to proactively confront the threats that once led to global catastrophe, giving him the chance to rewrite history. His objective is to dismantle the Box of Malice before it can inflict its destruction, utilizing his foresight to protect those he couldn't save before and to alter the course of fate.

Interestingly, "The Regression of Great Sage Riddle" originated as a manga before being adapted into a "light anime." This particular animation style, sometimes referred to as a motion comic, directly converts manga manuscripts into animated sequences, often with voice acting, giving it a distinct visual feel different from traditional animation. The episodes are notably concise, typically around five minutes in length, making it a quick and engaging watch for viewers.