Undertale is a recently released RPG indie game. Its main selling point is that there's no enemy that you need to defeat in order to beat the game (there are both hack-and-slash and nonviolent ways to end every encounter). There's
a freely available demo that basically represents how the full game plays up until the first boss; for both the demo and the full game, there are multiple endings and multiple variations of dialogue that you may encounter in subsequent playthroughs.
The game is very unique in many respects, and while the gameplay is fun, the greatest value in the game is that of the experience itself. For that reason, I'd suggest playing the game (whether you play the demo or full version)
before looking stuff up on it (the reviews linked on the Steam page are reasonably spoiler free, if you'd like to read them first). You'll appreciate the untainted experience.
(*edit*) The forums support the spoiler tag; you can put blocks of text in [spoiler=optional title text] and [/spoiler] tags to discuss spoilery content without spoiling others on it.
