Luke Skywalker and the Shadows of Mindor first established the Nihil Retreat as "twelve stellar clusters within" the Perann Nebula, however, The Unknown Regions establishes the Nihil Retreat and it's twelve stellar clusters as forming "a protective bulwark around the Perann Nebula," as well stating the Retreat contains only twelve planets. The 2015 update to the Star Wars: The Essential Atlas Online Companion simply stated the Perann Nebula to be located within the Unknown Regions. Lucasfilm has yet to provide clarification for this discrepancy.
- The Unknown Regions
- Book of Sith: Secrets from the Dark Side