In the age of "Shinobi(Ninja)," 5 superpowers were always at war, using a magical power called "Chakra." Chakra is the internal energy of people, and with Chakra, humans can use some "Ninjutsu" (e.g., cause thunder, heal injured people.)
You are one of the best Shinobi in the world. Your brother was killed in the war, and at the end of his life, you promised him to create a world without war. After tens of years of experimentation, you invented a Ninjutsu called "MugenTsukuyomi," which, once triggered, shows a never-ending illusion to every person until they die. In the illusion of Mugentsukuyomi, people can live in the world they want to live.
While you spend time on experiments, the world has changed. Because of massive casualties in the war, 5 superpowers made an armistice and shifted to using diplomatic efforts to solve a conflict. Mugentsukuyomi needs a large amount of Chakra at once to trigger, and to fulfill the requirement, you have to kill thousands of people.