[Gandalf Parker]

When an air mage reaches that province they will have a new command available on their orders list. Besides things like Defend Province, Move, Patrol Province, Blood Hunt, Construct Building; there will be a new one for Enter Site. There is no visual change but the command stays there as the active command so you know who is in the site. Once entered, the commander stays entered until you change it.

There is no advantage to powerful mages versus little apprentices. So if it says "Air Mage" then just try to find a 1-air mage of low research, and young so you can leave them a long time (click on fatigue on the units view to see their age).

Usually they get one gain per turn. It usually ends up in the province queue so that you have to use the Army Setup screen to assign them to someone. Unless the mage already has followers in which case it assigns them to the mage. Its usually best to allow them to go to the province queue because once the mage "fills up" their leadership limit you wont get any message. Any additional units gained will just be lost until you notice and fix it. So in most cases dont give that mage any personal followers until you are ready for them to fillup and move out.

Some creatures gained require mages of that type to lead them. In that case you have to plan for moving mages in and out of that province. Some creatures can be led by anyone so you can just plan for your armies to move thru that province to pick those up on the way, or even purchase a commander there whenever you have a bunch to move.