The Tower of Eternity is the physical representation of the god Aion. At one point it connected the two halfs of Atreia, but was destroyed by the Balaur, separating the two halves and causing spite between the two worlds.

This is the main cause of the PvPvE in Aion.

The Balaur offered a peace treaty but through a betrayal (that has not yet been elaborated on), the Balaur were led to attack the Tower of Eternity and destroyed it, causing a big catastrophe that brought destruction upon the world and split the Tower of Eternity into two pieces. Now Atreia is separated into two halves with one remaining half of the tower on each part of the world.