Kuro no Shishi

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 explaination.

(Source: MU)