Undertale is a 2015 2D role-playing video game created by American indie developer Toby Fox. The player controls a child who has fallen into the Underground: a large, secluded region under the surface of the Earth, separated by a magical barrier. The player meets various monsters during the journey back to the surface, although some monsters might engage the player in a fight. The combat system involves the player navigating through mini-bullet hell attacks by the opponent. They can opt to pacify or subdue monsters in order to spare them instead of killing them. These choices affect the game, with the dialogue, characters, and story changing based on outcomes.
Outside of artwork and character designs by Temmie Chang, Fox developed the entirety of the game by himself, including the script and music. The game took inspiration from several sources, including the Brandish, Mario & Luigi, and Mother role-playing game series, bullet hell shooter series Touhou Project, role-playing game Moon: Remix RPG Adventure, and British comedy show Mr. Bean. Undertale was originally meant to be two hours in length and was set to be released in mid-2014, but received delays.
The game was released for OS X and Windows in September 2015. It was also ported to Linux in July 2016, PlayStation 4 and PlayStation Vita in August 2017, the Nintendo Switch in September 2018, and Xbox One in March 2021. The game was acclaimed for its thematic material, intuitive combat system, musical score, originality, story, dialogue, and characters. The game sold over one million copies and was nominated for multiple accolades and awards. Several gaming publications and conventions listed Undertale as game of the year, and others have since listed it as one of the greatest video games ever made. Two chapters of a related game, Deltarune, were released in 2018 and 2021.