So in a nutshell, it's 75% castlevania, 25% minecraft. It's a 2D platformer, too, and has online servers you can set up. You can explore limited randomly-generated maps, dig up blocks and re-use those, and craft new items with the resources you have. That part is like minecraft (aside from the limited part). There's also tons of monsters to fight, the ability to summon bosses to fight, NPCs that move in to your base when you build rooms for them, and ba-jillions of items, along with a bunch of other stuff I forgot to mention. It's release date is tomorrow as of this post's writing, and will be on steam for $9.99 for the first week or so. The developers (Redigit and Blue) are planning to keep releasing new content every week, or around that time.
An Untitled Story is amazing.