The Aion is said to be a gate between the circle of ordinary time into the infinite realm of the soul.
Not commonly known and many who do know take it as a metaphor for "death".
There once was a painting, lost long ago from the The First Library that depicted the Aion as a large stone gate with a lion-headed being in the center. The being had four wings and stood on a globe encircled with runes representing years, months, and the ages that where and the ages to come. Two keys dangled from the lion's mouth. A sliver key and a golden key.