Chikyū Shōmetsu!! Bū Jāku e no Gyaku-Henshin (Earth Disappears!! Buu's Reverse-Transformation of Evil).

Available on Crunchyroll as "End of Earth". Plot Summary

After Vegeta removes the "Good Buu" from Super Buu's body, the villain regresses into his original, most dangerous state: .

Kibito Kai explains Majin Buu’s history, revealing that his previous "fat" form was actually a result of absorbing the peaceful Grand Supreme Kai, which had suppressed his pure evil.