Tsw'ells, enormous beings with tentacles, made their home in the seas of the planet known as Luprora. During Captain Phasma's pursuit to locate Lieutenant Sol Rivas (the hunt), she came across these tsw'ells. She killed one using her knife. Later, she utilized electrocution to eliminate several of them when attacking the R'ora stronghold.
The tsw'ells were native inhabitants of the oceans found on the planet Luprora. As the ocean waters rose, the tsw'ells migrated inland, causing problems for the immigrant Lupr'or by harassing and killing many of the fishermen. The Lupr'or considered the tsw'ells to be powerful monsters. In 34 ABY, when Captain Phasma and TN-3465 journeyed to Luprora in order to hunt down Lieutenant Sol Rivas, they faced an attack from a tsw'ells. During their brief conflict, Phasma managed to slay the creature using a knife, which earned her respect from the Lupr'or.
It also seemed that the Tsw'ells had a symbiotic bond with the R'ora, an aquatic species that was indigenous to Luprora. Several of the creatures were guarding a R'ora stronghold located on an island within a lake near a village populated by the Lupr'or. With Phasma leading, the Lupr'or used generators taken from a derelict starship to electrocute the tsw'ells in the lake, resulting in their deaths. Because of this, the Lupro'r were able to raid the R'ora stronghold, which then allowed her to kill Rivas in order to hide her actions related to the destruction of Starkiller Base.
The tsw'ells made their initial appearance in Star Wars: Captain Phasma. This was a comic series with four issues, released in 2017 by Marvel Comics as part of the promotional campaign for Journey to Star Wars: The Last Jedi. Kelly Thompson conceived the species, Marco Checchetto provided the illustrations, and Andres Mossa was responsible for the coloring.