Before I tackle the monster that is the Aeon there, I'm going to post my review of the Achaierai.
Achaierai:
[spoiler]The achaierai should be a simple creature with simple wants. The bird looks and acts as a normal "bird of prey" and, again, doesn't feel very PC-like. Perhaps as a mount of sorts or a trained beast for a druid. Unfortunately, the MM portrays them quite differently. They are agile, intelligent creatures, much like the raptors of Jurassic Park but with the mind of a cat (chase after it, knock it down, gore it up a bit and play with it before finally consuming).
The Hit and Run ability feels a little weak when compared with, say, the Anthropomorphic Animal which can "pounce" during a spring attack. Maybe allow its bite to start a grapple. Then, while it has a creature of one size or more smaller grappled, it flees with its prey (its beak maintains the grapple without the achaierai itself having to). This is especially effective if the creature moves in groups/flocks.
Long Legs is kind of a necessity for that creature's build. They have an ostrich-type feel to them. Disrupting Feathers would feel a bit on the tacked-on side, but they are outsiders. Outsiders seem to have this penchant of being resistant to magic (and, again, it matches with the MM version). Do the multiple legs grant a bonus on stability? Possible grapple bonuses?
Regarding Black Cloud, is the cloud considered to be "poisonous"/Poison? It is important to know for the sake of defending against its attack. Druids, for example, would be immune once past ninth level. The improved Maddening Cloud is a nice touch, though Will Saves are usually on the lower end as levels rise, making it rather powerful on its own.
Ability increases seem standard. One thing, though, that always got me was the defined "Lawful Evil" of the creature. If the creature were somehow of another alignment, would its natural/manufactured weapons count as the new alignment or STILL of the old alignment? Remember, there are always exceptions to a given alignment. I'm assuming you prefer to not look at alternate alignments as a possibility for simplicity sake. I can't say I fully disagree with you, but I think a simple clause could fix it right up.[/spoiler]