Dark World

A Dark World is an alternate reality inhabited by Darkners. Dark Worlds are brought into existence by Dark Fountains and exist within various areas of Hometown (the "Light World"). Dark Worlds are where the majority of Deltarune takes place.
== Overview ==
Dark Worlds are powered by Dark Fountains, which an individual can create by projecting their "determination" into a sharp object and piercing the ground. When the surrounding environment becomes subsumed in darkness, the Fountain gives form to a Dark World.
Each Dark World represents the contents of the Hometown location where the Dark Fountain was created. For example, the Hometown location of the Card Kingdom Dark World is the unused classroom. The classroom contains a rug with a similar color to the Field, a checkerboard corresponding to the Great Board, and a cupboard filled with playing cards similar to Card Castle.
Several Dark World objects have much more mundane Hometown equivalents: Kris's weapons correspond to pencils or an Eraser, and the Shadow Crystal corresponds to a shard of Glass. However, some objects, such as Noelle's Silver Watch and the Egg, directly match their Hometown counterparts.
Dark Worlds are populated by beings known as Darkners, who call the inhabitants of Hometown/the Light World "Lightners". As with their surroundings, many, if not all Darkners correspond to items found in a Dark Fountain's immediate vicinity. For example, Top Chef appears to be the personified version of a spinning top, Seam resembles the purple stuffed animal lying against the back wall of the unused classroom, etc. In addition, Spamton is believed to represent spam emails, which suggests Darkners do not have to be based on physical objects.
Darkners only reliably persist within the Dark World from which they originate. Though a Darkner can visit Dark Worlds other from where they originated, they risk turning to stone (possibly due to them being "incompatible" with the theme of that world), although they will make a full recovery when returned to a compatible Dark World. The Closet Dark World is the one exception to this, since its Dark Fountain emits pure darkness, meaning Darkners hailing from any Dark World will persist there.
Lightners, on the other hand, can persist in any Dark World and can travel to multiple Dark Worlds without consequence. They can visit Dark Worlds by entering their corresponding Hometown locations. Lightners have only been able to leave Dark Worlds via two methods: by sealing that world's Dark Fountain, and by using the Castle Town entrance. Darkners can be transported between Dark Worlds by Lightners moving their Hometown counterpart objects.
So far, Darkners appear incapable of opening Dark Fountains, since Queen sought out Noelle as a means to open additional Fountains. Moreover, it is unclear if a Dark Fountain can be created inside a Dark World, or what effect that would produce (though Berdly almost attempted this at the end of Chapter 2). However, Dark Worlds can directly connect to each other if their Fountains in Hometown are physically adjacent, as shown with the Closet Dark World and Card Kingdom.
Curiously, Ralsei, Jevil, and Starwalker, who are presumed to be Darkners, are able to persist in other Dark Worlds without consequence, unlike other Darkners. Ralsei can also travel from one Dark World to another without the assistance of a Lightner. It's possible Ralsei is able to persist because he's from the Closet Dark World; since this Dark World's fountain is one of "pure darkness", this might mean Ralsei is "compatible" with all other Dark Worlds. 
If Lightners receive harm in the Dark World, this is reflected in Hometown. This is reflected in the fate of Berdly based on what happens to him in the Cyber World. If Queen is defeated through violence in her battle, Berdly removes the plug on his face himself, electrocuting and burning his right arm; when Berdly awakens in Hometown, he proves unable to move his right arm. Similarly, if Berdly is defeated by Noelle's "Fatal" spell, he does not wake with the other Lightners after the Cyber World's Dark Fountain is sealed.
== Main Story ==
=== Chapter 1 ===
Kris and Susie first enter the Closet Dark World through a supply closet at their school. This Dark World contains the one and only pure Fountain of Darkness. After defeating King and sealing Card Castle's Dark Fountain, Kris and Susie return to Hometown.
=== Chapter 2 ===
The following day, Kris and Susie revisit the Closet Dark World. After meeting up with Ralsei and Lancer, allowing the inhabitants of the Card Kingdom to live on the Closet Dark World. However, as a result of the sealing in the previous chapter, the Card Kingdom can no longer be visited.
Later on, Kris and Susie (along with Berdly and Noelle) go study at the library only to find that it, too, harbors a Dark Fountain. This Dark World is ruled over by Queen. After defeating Queen, and sealing the Dark Fountain in her mansion, Kris and Susie return to Hometown once more.
In the middle of the night, Kris creates a Dark Fountain in Toriel's House, turning it into a Dark World.
== Sublocations ==
These are the currently known locations within the Dark World, ordered by when Kris and their party visits them.
* Closet Dark World (School: Closet)
** ??????
** Castle Town
*** Ralsei's Castle
* Card Kingdom (School: Unused Classroom)
** Field
*** Great Board
** Forest
** Card Castle
* Cyber World (Library: Computer Lab)
** Cyber Field
** Cyber City
*** Trash Zone
** Queen's Mansion
* Unnamed Dark World (Toriel's House)
== Trivia ==
* Using the Cell Phone while in a Dark World does not work, and plays an unskippable audio cue. The same audio cue was used in association with W. D. Gaster, a hidden character in Undertale.
fr:Dark World
pl:Dark World
pt-br:Mundo das Trevas
ru:Тёмный мир
vi:Thế giới Hắc Ám
zh:黑暗世界