Is it related to Samurai Showdown??

Samurai Shodown was kind of based on this. It is a movie from the seventies connecting all these characters together (correct me here on the date, or even if it is only a movie).

Some of the characters really existed such as Musashi, the monk Hozoin, Yagyu Jubei and Amakusa as well... not exactly like in the movie though...

Someone else can provide more background on it perhaps...

Actually, if anything, I think it's more related to the horrifically bad anime Ninja Resurrection... Which also has the culture hero Jubei Yagyu as the lead character. And the titles are similar enough and the plots similar enough, I'd guess...

I doubt I'd recommend watching EITHER flick...

They're all loosely based on the same novel, with Samurai Shodown being the loosest part of it. For the record, this is the second live-action remake. The first movie was a Sonny Chiba flick from the late 70s, then there was a straight-to-video series in the mid-90s (when there were tonnes of s2v titles being released), and now there's this new one. The anime came out around the same time as the straight-to-video series.

Also, for the record, the novel was written by the guy who wrote the Koga Ninja Chronicles, which the anime Basilisk and move Shinobi were based from. Oddly enough, that novel was the inspiration for the novel that Ninja Scroll was based from.