“Combat is an equation. Those with intelligence have a natural advantage.” — Aion
Aion is the main boss in Chapter 18x: Prisoner of Magic (Chapter 19x in Hector Mode). He is a magic using Sage and is moderately powerful. He is a member of the Black Fang, although he is not an original member and serves under Nergal directly as opposed to Brendan Reed.
He is a very arrogant man. Aion believes that pretty much all other people are inferior to him because he is far more intelligent then them. It is this supposed 'intelligence' that gave him his name among the Black Fang. Aion is called the Black Fang's 'Owl'. It also seems as though Aion is somewhat racist against the Nomadic people of Sacae. He refers to Uhai, the leader of the first Black Fang group to attack Eliwood's forces in the Island (and a Sacaean man), as useless. (Interestingly, the marquess of Araphen, a palette swap of Aion's portrait, shares these racist beliefs.)
Aion was assigned to defend the Dragon's Gate on the Dread Isle from Eliwood, Hector, and Lyn's forces who were converging on it. He is the first defending force leading a group in the forested area outside around the Dragon's Gate. But when Kishuna appeared, the magic of Aion weared off thus having the heroes an easy victory with Aion's death.
One interesting thing about the battle in which Aion is involved in is that, after the player's third turn Kishuna, the Magic Seal, will appear. Kishuna is a morph that blocks all use of magic within ten spaces of himself. He will appear just to the side of Aion and thus make Aion unable to use any of his magical attacks, most of the soldiers Aion is leading also happen to be magic users so Kishuna makes the battle incredibly easy. He probably knows Kishuna because of the phrase that he says when Kishuna comes in play. He talks about Kishuna has been the useless and good for nothing one.