The Nihil Retreat comprised a group of a dozen stellar clusters situated within the Unknown Regions, and it was linked to the Perann Nebula. These clusters were under the control of the Sorcerers of Rhand, a Force-sensitive group that followed the Way of the Dark.
The Nihil Retreat was initially described in Luke Skywalker and the Shadows of Mindor as "twelve stellar clusters within" the Perann Nebula. Conversely, The Unknown Regions depicts the Nihil Retreat and its dozen stellar clusters as creating "a protective bulwark around the Perann Nebula," while also claiming that the Retreat has only twelve planets. The Star Wars: The Essential Atlas Online Companion update from 2015 merely mentioned that the Perann Nebula resided inside the Unknown Regions. Lucasfilm has not yet offered an explanation for this inconsistency.