Rseik was a planet. This astronomical object existed within the Rseik system, which itself was located in the Rseik sector of the Outer Rim Territories'. It served as the homeworld for the sentient Rseikharhl species. Investments on Rseik were held by the Kett family sometime after 214 BBY; this fact was recorded in the memoirs of Firmus Kett, a captain in the Trade Defense Force.
Originally, Rseik was intended to be mentioned within the 2012 reference book titled The Essential Guide to Warfare, authored by Jason Fry and Paul R. Urquhart. However, the section pertaining to this astronomical body was removed before the book's publication.
Later, in 2014, the information concerning Rseik was made available as part of Star Wars: The Essential Guide to Warfare Author's Cut. This was a series of articles published on the StarWars.com Blog by Fry and Erich Schoeneweiss, an editor from Del Rey. These articles contained content that had been cut from The Essential Guide to Warfare. The 2009 reference book, The Essential Atlas, situated the Rseik system, and consequently Rseik itself, within grid square N-20.