Undertale is a Role-Playing Game by Toby Fox, released on September 15th, 2015. Its development was the result of a Kickstarter campaign that raised $51,124. The game and a demo are available on its main website here, and trailers can be viewed here and here. A port for the PlayStation 4 and Playstation Vita was released on August 14, 2017, while a Nintendo Switch version was released on September 18, 2018.
The game tells the story of a young child who investigates a mountainside cavern and falls down a deep hole, awakening on a bed of flowers in a dark and mysterious place. Now the child must find a way back home, encountering all manner of creatures along the way.
Undertale borrows heavily in atmosphere from the MOTHER trilogy, and uses combat mechanics similar to Bullet Hell games, where you must dodge enemy projectiles and protect the heart-shaped avatar representing your SOUL. The game's story changes significantly depending on how you deal with the monsters you encounter: you can fight and kill them to earn EXP and increase your LV, or figure out how to befriend them and show them mercy.
Before reading the tropes section or other sections, please note that by its very nature, Undertale is very difficult to talk about at length without spoiling it. Read with care.