Fist of the North Star: Raoh Side Story Junai Arc

Kenshiro is the heir of the legendary martial arts known as Hokuto Shinken, which can kill a man with a single blow, by striking the pressure points in the enemy's body. Hokuto Shinken can be passed only to one successor at a time. When he was chosen as the successor, he parted ways with his brothers, Toki and Raoh. The former decided to use what he knew of the art to cure people... while the latter has chosen to use his supreme strength to install peace in the world by power.
Kenshiro is accompanied by Lynn and Bart, and the movie's story begins when an overlord starts kidnapping children to use them as slaves for building his own pyramid...

(Source: AniDB)