If you have bee living under a rock and don't know what Undertale is allow me to explain without spoiling the game.
Undertale is a game inspired by Earthbound and made in GameMaker Studios, it's creation took 2 years and only had one developer (Toby Fox)
It's battle system is a mixture of RPG and Bullet Hell, this makes for a very fun combonation that can be pretty addicting.
However, like most good RPGs that is not all it has going for it, it's story is pretty great and has many lovable and iconic characters. (I'd be surprised if you've never seen a picture of a certain skeleton), the game allows you to take many routes of who and what you spare and do not spare, sometimes you really have to think to spare someone and other times it isn't so hard, killing is a lot easier. The graphics are nothing special but they were a good sacrifice to make a great battle system and great story, the music is INCREDIBLE and are about as iconic as the characters you meet.
I cannot tell you the story itself seeing as it is much better to play it for yourself, I recommend playing it multiple times for different endings.
I'd rate it 9/10 (No game is a perfect game, that is my main thoughts when judging something so if I really like it I tend to give it an almost perfect score)
[If you have played the game the do not post spoilers in the reply section please, have a good day.]