The Expanded Universe is a collective fictional universe containing the entire Battlezone series and additional works created by its fan community which between them tell a single, coherent story.
Fan expansion of the Battlezone universe began with the release of Battlezone, which featured a built-in Map Editor and allowed users to create their own Multiplayer maps. The release of the Instant Action Demo, which demonstrated the use of the Map Editor to create Instant Action missions, allowed fans to develop single-player maps of their own design; by the end of the next year, fans had begun to release sets of levels as campaigns. Since 1999, several of these packs have been released, with Last of the Galilean, Shrieking Eagles and the Red Wolf Mission Pack being amongst the most well-known.
Having seen the amount of mods fans had made for the first game, Pandemic Studios deliberately set out to make it easier to develop mods for Battlezone II. As with Battlezone users began releasing their own maps almost immediately, but in mid-January Brad Pickering created a pack of example mission scripts to help developers create their own single-player missions. These were used to some extent, but single-player scripting reached full popularity once Bob Stewart released his DLL Scriptor.
The expanded universe first came to prominence - and first began to coalesce as a single entity - with the release of Forgotten Enemies, the first Community Project. It served as an introduction to expanded works to many of the series' fans, and its quality meant that it is widely considered a true sequel to the series. Since its release it has been considered the base of the expanded universe's content.
As well as dozens of mods, the expanded universe also contains several works of fiction, including the The Battlezone Chronicles, Uncle Avatar's Storytime, The Fall of the Old Gods and the short story Lost Legacy.