In addition to some new capturables, turrets, and other ships that are available in any game once this expansion is installed, there are two major new factions. The Scourge is your greatest foe, the smartest and most dangerous sub-group of the AI. The Fallen Spire is your greatest ally, providing you with unprecedented firepower.
You can use both factions, either, or neither in your campaigns as you wish. Thanks to lobby customization options, you can even force the Scourge to be your ally against other foes, if you prefer. The Scourge are an optional faction that can be enabled via the lobby, or by activating a beacon in any game.
The AI felt a need for a subsidiary force to wipe out small annoyances, and not need to bother its main Processors with petty things. The AI took various races they have conquered in the Arcenverse, twisted them by genetic engineering, and fused them into a new and powerful force to crush its foes. The primary races the Scourge use are the Thoraxians, the Peltians, the Neinzul, the Evucks, the Burlusts… and even Spire. The Fallen Spire are an optional faction that can be enabled via the lobby, or by activating a beacon in any game.