The Chaos Moon, in theory, is the rare event in which the three moons of Dawn, Saexor, Deztine, and Sethamtic, line up in a linear position in which Saexor, the largest, is closest to Dawn. The gravity from this lunar pattern is so strong that it pulls on all mana within the Great Barrier, causing all mana to freeze. Of course, this is all only theory and may be disproven.
The first Chaos Moon's exact dating is unknown, but Snow Elven records narrow the timeframe down to around 590. The Snow Elves exact words were. "As almost six fathers have passed, tonight the glorious moon of Kryossiom, Saexor, has gained an azure hue and it seems as Saeomi has cursed us, for the gift of Kryossiom no longer stands. May Kryossiom forgive us."
The second Chaos Moon happened on 3562. It's occurance was vastly studies by scholars across Dawn, namely by the Order of Forbidden and Forgotten Magic, who's theory is widely accepted by scholars.