An indie RPG created by
Toby Fox that has sold extremely well and seems to have taken over the Internet.
The game follows the story of a human child who has fallen into The Underground, a society filled with monsters after humans sealed them away in a war long ago.
In battles, enemies attack your soul (which looks like a heart) in a bullet hell-like method, and you can either fight them, or via the ACT and MERCY buttons, figure out how to spare them.
The game's story is affected by how many monsters you kill, as well as which 'Boss Monsters' you kill.
It can be hard not to stumble across spoilers of this game when you are
looking into it, as many popular characters are considered 'Walking Spoilers'.