For an unbeknownst amount of time, the world has gone through a cycle of being consumed and reborn at the breath of the World-Eater. This fact is unknown to most living within the realms.
Though much of the previous cycle is lost, some ruins linger beneath the surface of the world, and some knowledge leaks through to the next. Examples include The Knighthold and the great stretches of tunnels reaching across Tav’esh from the Undercity of the Second Cycle.
The current cycle is commonly referred to as the Fourth Cycle.