New factions and battlefields join the world of Root!

The Underworld Expansion includes:
* The Great Underground Duchy: an imperial faction that mixes the flexibility of the Marquise with the escalating Eyrie Dynasties.
* The Corvid Conspiracy: a secretive faction that hatches plots directly into the hands of their opponents.
* Two new maps: dig tunnels on the mountain map or fight to control the ferry on the lake map.