Ninja Spirit is part of a remarkably rare breed: it's a TurboChip that even the most adamant TG-16 detractors typically credit as being extremely cool. I'm going to go out on a limb and assert that its good reputation isn't attributable solely to the score of "10" bestowed upon it by Electronic Gaming Monthly's Sushi-X. I think we can ascribe the accolades to the dark, somber manner in which the game sets up and presents its hack-and-slash-and-jump-really-high action: the opening sequence is a scene of tragedy; and once the gloom sets in, it's never shaken. Well into the night, heavyhearted Moonlight makes a vengeance-driven trek through murky marshes and dimly lit temples as hordes of blade-hurling ninja, sneaky spear wielders, and solemn-faced giants look to deal the blow that evokes his death howl.
While exhilarating, the kill-heavy proceedings are hardly deep. It's as if Irem took the few alluring elements that old Legend of Kage has going for it (most notably the ridiculously high jumps Kage is capable of performing) and placed them in a context where the fundamentals are actually sound. Some of Moonlight's end-level adversaries are embarrassingly easy to beat, but most of them are memorable design-wise, and the music that augments the showdowns is wonderfully fast and intense. In fact, the moody Far Eastern-flavored soundtrack is superb on the whole and establishes a very appropriate sense of unease.
Multiple always-in-hand weapons, gravity-defying ceiling jaunts, and recruitable Moonlight-mimicking spirit-allies are other aspects that contribute to Ninja Spirit's high fun factor, which renders its lack of difficulty forgivable (note that its "Arcade" mode is significantly tougher than its "PC Engine" mode and does feature an extra ending scene). As far as old-school ninja games go, I'd say NS doesn't come close to the amazing Shinobi III but can hold its own against any of the rest.
NEC was very proud of this big boss back in the day. He appeared in lots of ads and preview/review screens.