The Imperium refers to the overall body of works Archengeia has been developing since grade school. The entire Imperium shares one contiguous continuity, and there are multiple over-arching plots and stories that stretch across multiple existing settings as well as his own.
The Imperium contains a large amount of fan fiction, by definition, as it stretches across existing settings in addition to the new one of the Imperium itself. Not all fictional worlds or places are within, but quite a few are. A few notables include Star Wars, Dragon Age, Mass Effect, The Elder Scrolls, Warcraft, Starcraft, Diablo, Final Fantasy, the Mana series, Breath of Fire, and many others. Most often these stories take place as smaller gaiden stories of the overall Imperium, rarely affecting the Imperium's over-arcing plot (with a few notable exceptions), instead serving as character examinations of the Bastion Agents or personnel assigned to such a region.
The Imperium gets its name from The Empire, a vast multi-galactic organization which grew out of the chaos of the Original War and spread during the Arisal to the heights of its power it now maintains. The Empire has weathered multiple disasters in its time, including the Hatred Debacle and the Swarm War. The Empire's level of magical and technological advancement is extreme, providing them near total dominance of all known space.
While Archengeia has written out most of the overall Imperium storyline in his head through the entirety of what he refers to as the first Act, on paper the stories have only just passed the Swarm War and most of his active stories are dealing with the consequences of that event. Four major stories surrounding the Swarm War include The Primes, Extant, Eilimenti, and Primus.