The Syrons are the original inhabitants of the Shadow Realm, from when that world was clean and filled with light. They explored the cosmos, employing the powers of magic to perfect their world. Then the Shadow Demons came.
For thousands of years the Syrons battled the Shadow Demons, but over time their exhausted world was consumed in darkness. For generations, the Syrons were enslaved,used as food and forced to attend to their captors needs as the demons slumbered and waited for a new world to consume. Every time they attempted to break free, they were met with unfathomable horror and torture. As a result, the Syrons live in a fog of hopelessness, expecting nothing but a life of misery. Their once strong spirits bear the weight of a thousand generations of doomed children.
The Syrons may possess the power to defeat the Shadow Demons, but lacking the ability to dispel their dismal past, the Syrons will remain as trapped food to grow a new batch of hungry monsters that are eager to enslave another world.
--Shadow Magic Manual
At present, there is no information on whether the Syrons will return as a playable race.
While some of the more imposing members of their race visited Athla during the time of Age of Wonders 1, the majority of the Syrons arrived on Athla as slaves and refugees during the war against the Shadow Demons. With assistance from Athlan wizards, they were able to break free of the yoke of the Shadow Demons and participated in the events leading to the defeat of the All-Devourer.
Apparently native to the Shadow World (from a time when that appellation was not entirely accurate), the Syrons joined the Archons in leaving Athla, seeking to reclaim their world. Events since indicate that this effort may not have gone as they had hoped, and the fate of the Syrons remains unknown.
The historical Syron unit lineup can be found on the Age of Wonders 2 Heaven site: http://aow2.heavengames.com/aowsm/gameinfo/units/syrons.shtml