Just by looking at the screenshots, you'll be able to figure out what drew me to this game: it looks amazing. Everyone loves it when sprite characters are put in a polygonal world, but it's a concept that's mostly absent from modern indie games, even now that games with a Playstation-inspired look are becoming ever more popular. The game itself is a nice little platformer, too, which is also a positive.
Though, "nice" isn't the best word to describe it for a few reasons. As the title suggests, it's about a massacre (perpetrated by you) on an island inhabited by demons. The demons themselves are all weird, mangled mockeries of humanity at best, plus they burst into big pixelly bloodsplatters on death, too. To add to the horror, there's at least one enemy that's a direct reference to Splatterhouse 2, one of the all-time most shamelessly gory games there's ever been. (The enemy in question is the partially-rotted zombie, who in both games crawls around as a legless torso after you hit him for the first time).
As well as Splatterhouse, Castlevania is also a clear influence on the game, with some of the tracks sounding like (very good) cover versions with more of an asian horror flavour thn Castlevania's European gothic stylings. Plus, you get ammo for your special attacks by attacking lanterns that are floating in place in the stages. It does also have its own style, too, with a few different attack options. You have you standard attack combo, you can press up and attack for a shoryuken-style attack, press down and attack for a sliding kick, and press down and kick while in the air for a downwards attack that bounces you off of enemies really high.
So, the meat of the game is navigating the platform stages, figuring out the best way to get through each enemy encounter while taking as little damage as possible and also not falling to your death. Like you'd expect from a combat-focussed action platformer, I guess. You really do have to think about every encounter, and approach them cautiously, too, as the game is completely merciless. Death means getting sent back to the last checkpoint, which are few and far between, and you really can only take a few hits before that happens. On the plus side, there isn't really a time limit, and most enemy types don't become a threat until you get close to them, so you can stand still and appraise the situation at your leisure.
Demon Island Massacre is definitely not a game for the casual player, or for the crybaby. But it is aesthetically beautiful, and it's fun and rewarding if you're able to engage with it on the level it demands. If you're attracted by the way the game looks, and you think you can at least attempt to play it without writing it off as "too hard", then you should definitely give it a try. There's a free play-in-browser version on the game's itch.io page, as well as a pay-what-you-want download version, and it's defintely worth your time, at the very least.