Adding a new faction to Endless Legend [official site] must have been a daunting task. Amplitude's 4x masterpiece is built around the concept of highly individual playstyles and rulesets for each of its factions and if the newcomers didn't live up to what had come before, they'd look terribly out of place. That's why the just-released Shadows expansion doesn't stop at adding a new faction, going by the name The Forgotten, it also adds a whole new layer of systems to flesh them out. Espionage, counterspying and stealth trickery. Early word is positive.
The espionage system sounds like it'll change the flow of the game dramatically. Rather than simply tacking on a few extra unit abilities and city improvements, the possible presence of spies, assassins and traitors. This means you'll need to keep a closer watch on your settlements and activities within your own borders, rather than constantly looking to the horizon, the sky and the crumbling future.
The Forgotten are "nightmares from the past":
"...a faction that all others thought to be lost or dead. Against their will they were forged into a tight society of violent and distrustful survivors, having spent the final years of the cataclysm alone on the surface of Auriga.
"Now, as rifts in their society have led a splinter faction to declare its independence, the Forgotten are risking everything in the hopes that the other factions will accept them - or kneel before them.
"Masters of deception, infiltration, and sabotage, the arrival of the Forgotten among the factions of Auriga will change everything."
Their units can poison, assassinate and turn into mist. The espionage, infiltration and sabotage abilities aren't just for The Forgotten though. Heroes can gain skills that allow them to investigate empires from within, discovering their secrets and fomenting trouble.