Just bouncing some ideas
Death, Famine, War and Pestilence
Great opportunity for some great art work, interesting rules and interaction.
Death: nature, direct damage, all enemy troop bonus inverted X 3 i.e. a +10 attack becomes - 30
War: Red, triples troop bonus for allies
Famine: dark, all tiles count as weak damage
Pestilence: holy, the hp of one random enemy is given to all enemies. I.e if the enemy has 750 health remaining, all enemies match that hp.
I bet someone could think of some better ideas.