[Gandalf Parker]

Does that unit have age icon showing? (when viewing that units info is there a white picture of an old bearded man showing under the stats) That means he has gotten old. Its old age causing more and more problems until he finally dies from old age.

Some units are recruited already old. Also if I remember, fire magic increases it and nature magic decreases it so Abysia would be in trouble.

Click on the Fatigue line of the unit and it will give you more info about his age and age limits. In fact, you should click and right-click just about everything shown on the units display, and on the map menus, and any other display. You would be amazed at how much information is available.

There is no way to fix age easily other than ways to die and return. BUT there are some items that will "freeze" or slow aging. Before investing extra in a unit (such as giving it more magic or making it a prophet) you might want to check their age and pick one that wont get afflictions, then give it one of the age-locking items to protect your investment.