Sundered, the "replayable Metroidvania" with giant boss battles, is out now. In it, you slash and stab your way through a procedurally-generated 2D labyrinth, gradually unlocking new skills and abilities that help you delve deeper.
Developers Thunder Lotus (of Jotun) want it to be tough as nails—one of the elements of the "core gameplay" listed in the summary of the game is simply: "die". And when that happens you'll be hauled back to a central hub and have to set off again.
But there's a clear progression system, with success in a given area netting you items that will let you access new areas of the map. There's also a branching skill tree and "hundreds of upgrades" to play with.
I've been trawling through the early Steam reviews and the two most common words seem to be "beautiful" and "frustrating". That might just sum it up: if you don't mind dying an awful lot then it will reward you with some gorgeous environments and set pieces (those boss battles really do look epic).
Here's the launch trailer: