For example, the Hometown location of the Card Kingdom Dark World is the unused classroom. The classroom contains a rug similarly colored as the Field, a checkerboard corresponding to the Great Board, and a cupboard filled with playing cards similar to Card Castle. The Card Kingdom is inhabited by Darkners who correspond to the various items strewn throughout the unused classroom.
Darkners can only inhabit the Dark World from which they originate for extended periods of time. The exception to this is the Closet Dark World, because its Dark Fountain emits pure darkness. If Darkners do visit another Dark World, they turn to stone after remaining in it for too long, although they make a full recovery when returned to a compatible Dark World.
Lightners, on the other hand, are able to remain in Dark Worlds freely. 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, although so far only Kris has been shown to do this.
Curiously, Ralsei, who is presumed to be a Darkner, is able to travel from one Dark World to another without the assistance of a Lightner, as well as persist in other Dark Worlds without consequence, unlike other Darkners. It's possible this is because he's from the Closet Dark World, and may be compatible with all other Dark Worlds because of this.
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.
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; if he is injured this way in the Dark World, when he wakes up in Hometown he is unable to move his right arm. Similarly, if Berdly is defeated by the SnowGrave spell (which is labelled as "Fatal"), he does not wake up with the other Lightners after the Cyber World's Dark Fountain is sealed.
Kris and Susie first enter the Dark World through a supply closet at their school. This Closet 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.
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.
These are the currently known locations within the Dark World, ordered by when Kris and their party visits them.
- Closet Dark World (School: Closet)
- Card Kingdom (School: Unused Classroom)
- Cyber World (Librarby: Computer Lab)
- Using the Cell Phone while in the 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.