Il Leone Nero

One of Go Nagai's first works, this violent period piece takes place in the 1500s and stars a talented low ranking ninja named Shishimaru. In it, Shishimaru fights against Ginnai Doma (an immortal samurai who, despite being butchered multiple times, doesn't die) because Ginnai murdered his comrades. Despite being set in historical times, it has machine guns/lasers/robots/atomic reactors with no real explanation.

Note:
Scanlator is using the 1992 Aizouban 1 volume reprint by Chuko.

Cover images below:
4 Volumes (Complete, by Kodansha in 1978)
4 Volumes (Reprint, by Asahi Sonorama in 1984)
1 Volume (Aizouban, by Chuko in 1992)
3 Volumes (Bunkoban, by Chuko in 1995)
2 Volumes (Konbini-ban, by Shimanaka Shoten in 2003)