An exarch is a mortal, being, or creation, costing 3 PP, that you grant a portion of your divine power so that it may take actions for you. An exarch can provide a +1d4 combat bonus or generate 1 PP towards a specific keyword as though it were an artifact, and spending an exarch's PP counts against your number of Exarch/Artifact PP per week (2 max/week)
In addition to their mechanical bonuses, exarches provide something else as well. An exarch is typically an intelligent being, and is capable of roleplay as well as taking actions using your PP. Most valuable is that your exarch allows you to influence events when your god is not present, but the exarch is.
An Exarch’s benefits, effects, and capabilities are only limited by your imagination, but it is best to run unprecedented and highly powerful effects by an admin before actually posting them. Furthermore, if a PP generated and used by an exarch is not used in a way consistant with how that exarch would function, then the admins may declare the action invalid. (Example: My Exarch, floating through the Aether, cannot readily nourish a race living in the underdark.)