I'm really surprised this thread isn't already on here, at least I didn't find it by searching so I made it. The game's been out for about four and a half months now.
The summary without spoiling the game (really, I highly suggest you don't view the comments section for this or even beyond this initial post if you have any interest in playing; this game is heavy on that) is it's an RPG which you can either spare or kill characters (all called monsters) in. I played through 100% by sparing and highly recommend going that route, though I'm not about to say why in this initial post. I also haven't played again since my initial run.
Link to game trailer...
Undertale is available on Steam for $9.99 if you're just buying the game, the soundtrack is also $9.99 or you can buy both together for $17.99. Funny thing is the soundtrack's in a strange location on your computer if you buy both at once.
- Austin