Deltarune is a role-playing game created by American indie developer Toby Fox. The player controls a human, Kris, in a world 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 restore balance to the world. The players meet 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. The first chapter of the game was released for free to a positive reception on February 28, 2019 for the Nintendo Switch .
Deltarune is related to Fox's previous game, Undertale. Although several characters appear in both games, the setting is "not the world of Undertale". Chapter 2 is planned, but no release date has been announced yet.
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 choose attacks of 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. 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 can't 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 however, the three encounter Lancer, the King's son, 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. 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 the birdcage, but cannot do anything else. Kris draws a knife and turns toward the viewer. They smile, with their right eye flashing red.