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 as forming "a protective bulwark around the Perann Nebula." 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 Essential Atlas
- The Unknown Regions