Demon Sword plays a lot like Legend of Kage or maybe even Ninja Spirit in that you run like crazy and mow down enemies with your sword. Jumping propels you forward into the air turning your swordsman into a helicopter of death. You play as Victar and must gather all the piece of the titular sword to defeat the demon currently ruling the world. As you play through the levels and gather pieces, the sword gets bigger and more powerful.
In addition to your growing sword, there's also projectiles like darts. These obviously come in handy for bad guys just out of range. You'll acquire magic spells that come in handy when fighting level bosses. The bosses and bad guys are various demons and yokai in bizarre hellscapes. It's pretty awesome. As normal during the time, the U.S box art depicts more of a standard mid evil fantasy setting, however Japanese advertising gives a better idea of the actual game.
Speaking of difference between the American and Japanese versions, in Japan the game is longer and has more items/magic. I remember thinking I achieved the impossible upon beating the game that one weekend in the seventh grade. Now I wonder if I could repeat history with the Japanese version. Probably not. Demon Sword was one of those things that was MY thing. I'd draw (badly) the characters on my notebooks, and dream up potential sequels. I'm sure to this day though, that I've never said Victar correctly.