I just finished it yesterday. It's a beautiful Metroidvania sidescroller, which apparently was put together over 4 years by a loose international association of creative folks, who dubbed themselves "Moon Studios". (
Wikipedia page.) This is the hardest platformer I've ever stuck with till the end, and I played some doozies on the NES. After finishing it, I looked up the achievements list, and about fell off my chair when I read that one of them was to complete the game without dying. My stat for respawning from a soul link (user-created save point) is near 700. You learn by dying, and after a while, I'd jump to my death just so I could redo whatever sequence was giving me trouble, and end up with better health after it. There's another for completing the game without ever spending any ability points. Ouch!
Intricate, lovely visuals, haunting music, proper genre gating based on new abilities you find along the way (not to be confused with the abilities you can buy). Enormous world to explore, and lots of secrets to find. Highly recommended for whoever can stomach the sadistic difficulty. Let me elaborate a bit on that: The normal navigation is quite challenging, and full of death traps, but at least you can create your own save points to minimize what you need to repeat upon every death. However, the end of each major chapter is an escape from a collapsing area of some sort, where you are forced to move quickly, and there is no saving allowed. One major mistake or several minor ones reset the whole thing to the beginning. A few expletives at the screen, during and after, are advised. Stress relief.
Game's available on Steam and XBL (Xbox One).