The underground world is the home of monsters, many of whom challenge the player in combat players decide whether to kill, flee, or befriend them. Players explore an underground world filled with towns and caves, and are required to solve numerous puzzles on their journey. In the game, the player controls a child and completes objectives in order to progress through the story. Undertale is a role-playing game that uses a top-down perspective. Undertale employs a bullet hell/turn-based combat system in which the player controls the heart, avoiding attacks from enemies in between fighting, acting, healing, or mercy. Two chapters of a related game, Deltarune, were released in 20. Several gaming publications and conventions listed Undertale as game of the year. The game sold over one million copies and was nominated for multiple accolades and awards. The game was acclaimed for its thematic material, intuitive combat system, musical score, originality, story, dialogue, and characters. 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 released for Microsoft Windows and OS X in September 2015.

However, development was delayed over the next three years. Originally, Undertale was meant to be two hours in length and was set to be released in mid-2014. 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. Outside of some artwork and character designs, Fox developed the entirety of the game by himself, including the script and music. These choices affect the game, with the dialogue, characters, and story changing based on outcomes. They can opt to pacify or subdue monsters in order to spare them instead of killing them. The combat system involves the player navigating through mini- bullet hell attacks by the opponent. The player meets various monsters during the journey back to the surface, although some monsters might engage the player in a fight. 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. Undertale (stylized as UNDERTALE) is a 2015 2D role-playing video game created by American indie developer Toby Fox.

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