I'm pretty surprised that tehre is no Undertale in this forum already considering the whole world talks about it.
For those who don't know what this game is about, it's a rather unique game that blends different genres like RPG, Bullet hell, puzzle and some other stuff. It is heavenly inspired from the Earthbound/Mother series both visually and from the gameplay (at least at first glance) but makes soemthing very unique.
The game allows you to resolve combat without fighting. every monster can be "spared" an option that appears after you've solved small puzzles during the fight. Sparing means the monster will depart friendly from you. Overall this means you don't have to kill anyone in the entire game. But on the other hand you CAN kill everyone. The game features different endings depending on the decisions you make.
Battles are really interesting, they are a mix of turn-based RPG battles with bullet hell sections. On your turn you take a typical rpg action like fight, use an item or use an ability. On the enemy turn your character is represented as heart and the enemy uses different real-time bullet patterns against you that you have to dodge with your heart otherwise you receive damage. Especially the bossed have harder patterns that cannot be easily dodged.
I completed the game already twice and was very entertained with it. For a rpg it has a rather high replayability because of the different actions you can take. The problem is, I cannot tell much about the story because the game is best played when you don't know anything.
At least I can say that you play a human kid that drops into a cavern full of monsters and wants to find a way out.