ABOUT UNDETALE
Undertale has a deep, touching story. Long ago, humans and monsters lived in harmony. But war eventually broke out between the two. The humans won and sealed the monsters underground with a magic barrier, trapping them beneath Mt. Ebott...
After your first playthrough, you should have figured out that you are the seventh human to fall. The monsters only need one more HUMAN soul to free all of their kind. By just completing the game without taking your amount of violence or kindness into acount, you will get the neutral ending, where you escape the underground alone. Although, by sparing everyone and building friendships with the monsters, you can complete the TRUE pacifist ending; with all of the monsters being freed from the underground. Alternativly, you can choose to not spare a single monster, maxing out your LOVE(level of violence) and summoning the soul of the first fallen child to destroy the timeline.
:p