The Creation War was a terrible conflict of the Ergen Empire.
In The Chronicle
The Creation War occurred several thousand years before the time period of the narration. Histories from Caluptena exist in the Archives that are over two thousand years old, and Felurian states that the stage for this war was set long before men or fae.
According to Felurian, the war was primarily waged between two factions of creatures who practiced naming. One group, called Knowers, knew the names of all things, but lived in harmony with them, never exerting their mastery. The second group, known as Shapers, took the art of naming a step further to change that which they knew the true names of, creating many new and wondrous things. These very separate and different ways of life eventually led to discord between the two factions.
Seeking a refuge to practice their skill unhindered the Shapers created the Faen Realm. At the culmination of this feat, each Shaper built a star which they then placed into the empty sky. However, unsatisfied with only a star the greatest Shaper, Iax, used his ability to pull the moon into Fae, thus causing it to phase between both worlds. It was this act that sparked the Creation War, and according to Bast was done at the behest of The Cthaeh.
In The Name of the Wind, Skarpi explains how the Knowers were nearly defeated in this war, but during the battle of Drossen Tor, Lanre was able to conquer a great beast and the enemy was shut beyond the doors of stone. His death and subsequent revival by Lyra, bolstered the hopes of the Knowers allies, turning the tide of the Creation War. When the cataclysm had ended, Lanre proceeded to betray the Ergen Empire, destroying all but a single city. This act led to the creation of The Chandrian and The Amyr, and was also supposedly caused by the creature known as Cthaeh.
Speculation
Many believe the enemy shut beyond the doors of stone at Drossen Tor is Jax.