Shogun Assassin (1980, released by
Roger Corman's New World Pictures) was many Americans' (and other
English speakers) first glance at the excellent "
Lone Wolf and Cub" series of
Japanese sword films based on the
manga by
Kazuo Koike (story) and
Goseki Kojima (art)—the original stories taking up some 8000+ pages.
The film took a somewhat cut version of the second film in the series (1972's Baby Cart at the River Styx) and edited in some 10 minutes of footage from the first (Sword of Vengence, same year). Then there was English dubbing and added "narration" written from the Daigoro's (the titular "baby") point of view (there was no such narration in the original series). Some feel that the editing destroyed the artistic lyricism of the battle scenes (directed by the underappreciated Kenji Misumi) and that the new score is poor compared to the original.
Technically not the first introduction of the series, as the third film (known variously as Baby Cart to Hades and under the literal title Perambulator Against the Winds of Death among others; again 1972) was released under the title of Lightning Swords of Death by Columbia Pictures in 1974. Again with edits for violence and English dubbing. The intention was to capitalize on the then-current Kung Fu craze.
Interestingly, the Leonard Maltin review (surprisingly quite positive) refers to Shogun Assassin as a "sort of a sequel" to Lightning Swords of Death. Well...sort of.
(Sources: David Desser's essay in Reframing Japanese Cinema, 1992, as well as his class on Japanese Cinema at the University of Illinois; "Cult Movies" issue 12; notes included with the videotape of Baby Cart to Hades)