Deltarune is a role-playing video game created by Toby Fox. The player controls a human, Kris, in a town inhabited by monsters. Kris and a classmate named Susie fall into an unknown place called the "Dark World" where they meet Ralsei, who informs them that they are heroes destined to save the world. The trio meets various beings who call themselves "Darkners" during a prophesied quest to seal the duplicate Dark Fountain. Mainly through the combat system, the player navigates through different kinds of bullet hell attacks by enemies, which can be resolved peacefully or through violence.

Development of Deltarune began in 2012. The battle system was overhauled from Fox's previous game Undertale, resembling the one used in Final Fantasy games. The first chapter of the game was relesed for free on October 31, 2018 for Microsoft Windows and macOS; the Nintendo Switch and PlayStation 4 editions were released on February 28, 2019. A full release is planned, but no release date has been announced yet. The soundtrack, characters and sense of humor was praised by critics, with mixed reactions towards its similarities to Undertalea.

Like Undertale, Deltarune is a role-playing game with a top-down perspective. The player directly controls a human named Kris, but they may also select actions for other characters in the game. Similarly to Undertale, Deltarune includes puzzles and bullet hell sections in which the player must move a heart around a boxed area while avoiding attacks. Undertale's random encounter system has been removed; players can now see the enemies directly on the field, allowing for the possibility of avoiding them.

Combat is turn-based. Players can choose from a set of actions each turn such as fight, act, spare, using an item, or defend, which reduces incoming damage.[8] Brushing against attacks without touching them increases the Tension Points (TP) gauge, which allow party members to use spells. For example, Ralsei can pacify enemies by singing them to sleep. When a party member's hit points (HP) reaches 0, they become downed and will not be able to fight anymore. The HP of downed party members will regenerate automatically until it reaches 1; healing items or Ralsei's healing spell will also revive them.

While the goal of the game is implicitly to avoid fights and spare monsters, this is made difficult in that Susie—who is not initially controlled by the player—would attack enemies rather than spare them, and thus the player must also warn enemies of Susie's attacks if they want to show mercy.

Upon starting the game, the player is invited to construct an avatar. However, upon completion, the game deletes the avatar and informs the player that "No one can choose who they are in this world."

The player begins the story as Kris, a human child living in a village mostly made up of monster residents, with their adoptive mother Toriel, who is also a monster. Toriel drops Kris off at school, where Kris attends a class taught by Alphys. Kris and Susie, a delinquent monster classmate, are sent to get chalk for the blackboard. Upon entering the supply closet, both of them are pulled into the "Dark World". There, they meet Ralsei, a prince of the dark, who tells them that the three of them are heroes destined to close the Dark Fountain (a geyser of dark energy) to restore balance to that world. However, the King has seized control of the Dark World and is determined to spread darkness.

Susie chooses not to help, only wanting to return to her own world. Before she can leave, the three encounter the King's son Lancer, who tries to stop them from proceeding with various poorly thought out plans. Susie eventually decides to join Lancer, leaving Kris and Ralsei on their own. As Kris and Ralsei make their way to the King's castle, Susie befriends Lancer, and the four ultimately become a team. Upon realizing that they will have to confront the King, Lancer runs off to the castle and arranges for the King's henchmen to throw Kris, Ralsei, and Susie into the dungeon.

Susie escapes the dungeon and confronts Lancer, who explains he wanted to keep Susie and the King from hurting each other. Susie promises Lancer she won't hurt the King. Kris, Susie and Ralsei go to the top of the castle and confront the King in battle. Eventually, the King falls to the ground in exhaustion and Ralsei takes pity on him, healing the King. However, this is revealed to be a ruse as the King quickly incapacitates the three heroes, threatening to kill them all. If the player has resolved enemy encounters without violence throughout the game, Lancer turns the King's men against him and imprisons him, taking his father's place as king of the Dark World. Otherwise, Susie subdues the King using Ralsei's Pacify spell.

Kris closes the Dark Fountain so that they can return to their own world. There, Susie says goodbye, expressing interest in returning to the Dark World. The player can explore the town before having Kris go home to bed. That night, Kris shakes in bed, then falls on the floor and limps to the center of the room. Kris tears into their chest and rips out their heart-shaped SOUL, throwing it into a birdcage in the corner of the room. The player can move the SOUL around inside the birdcage, but cannot do anything else. Kris draws a knife and turns toward the viewer. They smile, with their right eye flashing red.

The idea for Deltarune came to Toby Fox in a dream he had in 2011 while he was at college, he saw the ending to a video game and was determined to create it. Fox was also inspired by a collection of playing card designs posted on Tumblr by artist Kanotynes, which were used in the final game. Development of the game started in 2012, although it was abandoned before Fox created the first room. Some music from the original project was recycled for Undertale, most notably the main battle theme (which became Papyrus' battle theme, Bonetrousle), and a song called Joker Battle (which was reused for the Toriel fight as Heartache). After the Kickstarter success of Undertale, Fox decided to make a game combining the two. Various factors such as the graphics, overhauled combat system and Fox's mental state make Deltarune a more challenging game to produce compared to Undertale.

Temmie Chang, who previously assisted Fox with character art in Undertale, served as the main artist for the Deltarune. She helped design characters, sprites and animations. Fox came up with Susie's design after having played Phoenix Wright: Ace Attorney. He originally based her off Maya Fey and she would act "nice and cute". However, as her design progressed, she eventually turned into more of a "thug". Fox stated that he planned to give an unnamed character a fire spell which they wouldn't be good at using, but decided against adding it in the first chapter.

Deltarune was developed by Fox in GameMaker Studio 2. The game introduces a new battle system comparable to the one used in the Final Fantasy franchise, contrasting with the original game's combat system (which shared similarities with that of the Mother series). Some of the music in the game remixes portions of the Undertale soundtrack, but most pieces are new. Unlike Undertale, Fox plans for Deltarune to have only one ending.

After previously teasing something Undertale-related a day earlier, Fox released the first chapter of Deltarune, a game "intended for people who have completed Undertale", on October 31, 2018, for Windows and macOS for free. Fox stated that this release is the first part of a new project, and considered the release a "survey program" to determine how to take the project further. Fox clarified that Deltarune will be a larger project than Undertale and once he anticipates getting a team to help develop and more, and when it is ready, will release the game as one whole package. Deltarune merchandise was announced on November 2018 in collaboration with Fangamer, including T-shirts, a poster, a plushie and a pin set.

A Nintendo Direct released on February 13, 2019, announced that the first chapter of Deltarune would be released on Nintendo Switch on February 28. On February 21, 2019, the official PlayStation Twitter account announced that a PlayStation 4 version of the first chapter would also be released on February 28. The Switch and PS4 versions were developed and published by 8-4. I hope you readed this, and i hope you did not cheat