[7] Similarly to Undertale, Deltarune includes both puzzles and bullet hell sections in which the player must move a heart-shaped soul around a limited area to avoid attacks.

Grazing attacks, defending, and hitting an enemy increase the Tension Points (TP) gauge, which allows party members to use spells or particular acts.

[10][11] The game begins with a voice prompting the player to create an avatar, but the "vessel" is ultimately discarded, as "no one can choose who they are in this world".

After being paired up with their delinquent monster classmate Susie for a group project, their teacher, Alphys, sends the two to get chalk from the supply closet.

Arriving there, they discover that a new Dark World has been created, ruled by a computer named Queen, who recruits Berdly to her side and attempts to do the same with Noelle.

Both parties are captured by Queen, but Kris and Susie escape, convince Berdly to change sides, and reunite with Ralsei.

Kris rips out their soul again, opens the front door, turns on the TV, and creates a Dark Fountain in the living room with their knife.

[12][13][14][15][c] The player manipulates Noelle to freeze enemies and solve puzzles by herself and makes a deal with Spamton to acquire a ring that improves her magic at the cost of her HP.

[16] Various factors, such as the graphics, an overhauled combat system, and Fox's mental state, made Deltarune a more challenging game to produce than Undertale.

[10][17] Temmie Chang, who previously assisted Fox with character art in Undertale, served as the main artist for Deltarune.

Fox stated that he planned to give an unnamed character a fire spell that they would not be good at using but decided against adding it in the first chapter.

[20] After previously teasing something Undertale-related a day earlier, Fox released the first chapter of Deltarune for free on October 31, 2018.

As he envisioned a larger project than Undertale, he said that he needed to form a team to release the full game, which would be sold as a single package.

He eventually concluded that GameMaker "still felt like the best fit for the project", and using the first chapter as a base, he began working in May 2020, with Fox and Chang joined by a few other team members.

[2] Fox has stated that Chapter 2 is perhaps the game's largest, due to it having the most cutscenes and its usage of a large number of characters, among other factors.

[28] In a 2020 update, he revealed that he was considering expanding his four-person team following wrist and hand pain, which had delayed production; this led them to receive over 1000 applications.

[35] Due to the ongoing length of development, Fox announced the following month that the initial paid release would only include the third and fourth chapters.

[36] He recruited a producer in the beginning of 2024, and his team set an internal deadline of September 1 for the completion of Chapter 4's main content, which was met.

[22] Although Mitchell Parton of Nintendo World Report thought that Deltarune "doesn't significantly change up the formula", he did not have a problem with it.

[42] Nintendo Life's Mitch Vogel was less positive, being disappointed that after how "fresh" Undertale was at its release, Deltarune ended up being "'just' more of the same".

"[41] Tarason found that the music had "a fresh new edge to it," whereas Parton described it as "emotional and solid" and expressed surprise at it being composed by one person.

[43] GameSpot's Michael Higham pointed out similarities between Deltarune and Undertale's music, believing that they're "callbacks to remind you that these two worlds are somehow bound together.

"[9] Video game music group Materia Collective released the official 40-track soundtrack, composed by Toby Fox and featuring composer-songwriter Laura Shigihara on the best-selling single "Don't Forget".

[44] Tarason liked the game's pixel art, calling it "more detailed and expressive" compared to Undertale, a sentiment that Parton agreed with.

[22][42] Higham further elaborated that Deltarune's ability to "communicate so much with so little" is one of its greatest strengths and that "character expressions and body language provide vivid displays of personality.

"[9] The gameplay was also generally well received, with Parton calling it "unique" and Vogel describing the combat as "an organic and well-implemented expansion of the original".

[42][22] Vogel was critical of the way that the Dark World was implemented, opining that despite it looking slightly better than Undertale, it "hardly feels like a cohesive or living place".