The Great Event is a term by which all life and civilisation is defined as the modern era. Dwarves and Elves refer to it as The Rending and The Judgement respectively.
The event itself comprised of a cataclysmic rupture upon not only the surface of the Talvir continent, but also - as modern scholars believe - in the very fabric of the universe, through which vast hordes of terrifying and never-before-seen creatures, known as The Swarm, flooded from what is now referred to as the Demonic Plain.
All that is known of the Great Event is derived from stories passed down through generation to generation and the few, mostly unintelligible and untranslatable, documents that survived The Crusade of Flames (35 A.G.E.). It is believed that the first appearences of the Tiefling race, at this time, are intrinsically linked with the rupture. It is also understood and believed to be the only time the races of man, dwarf and elf have ever been united beneath a common banner.