They shapeshift to gain new abilities. Each spot they take over keeps the bonus of the race previously there. Icon is just the ring, however spots they take over keep former icon.
Monks
They convert enemies to their cause; +1 troops added to stockpile for each region taken over with 5 or more units in it.
Warriors
They struggle through adversity and come out more prepared for victory. +25% shard bonus on lost games.
Spies
They appear in your color, so you need to watch out for them; the only distinguishing feature is on their ring (which matches their color rather than silver).
Ghosts
You can't see them until they're right next to you; other than the regions of theirs that you border, their regions appear to be empty spots.
It would be fun to play an all spies game
