You're gonna have a bad time.
Undertale is one of the most fascinating examples of video game narrative released in years. Because depending on which of two ways you play it, Undertale can either be one of the funniest, most good-natured, and kindest games about humanity and sympathy ever crafted -- or it can be a paralyzingly grim tale of mistrust, hatred, and the relentless slaughtering of innocent people.
If you've played Undertale before, you more likely than not played the Pacifist Route first, which is the way that the game was intended to be played. If this is the case, and by some additional miracle you've managed to avoid the game's overexposure on the internet, and then went to play the Genocide Run without knowing anything about it -- you are in for the shock of your life.
It takes the typical gaming standard of mindlessly ending the lives of enemy monsters and makes you feel like you really are the bad guy after all. All of the people who could have just easily been your friends, become obstacles in your path that you must crush to dust.
(Also, a brief but very important DISCLAIMER: DO NOT PLAY THE GENOCIDE ROUTE FIRST IF YOU HAVE NEVER PLAYED THE GAME BEFORE. Just trust me. The game will be ruined for you after this. Because the game will remember what you've done -- and it will not forgive you.)
Considering that pretty much everything about Undertale is best experienced without spoilers, and I'm already kind of doing that by talking about it at all, I'll just show you the game's trailer. That way you can decide for yourself whether or not this game looks like it has some of the most disturbing murder scenes ever put in a game hiding inside it.