The Cataclysm, as it is known in Correllendor, was the failure of a great magical experiment. It changed the face of Correllendor and the surrounding region forever.
The Seekers after Nothoreadhel created a network of gates and spatial magic which spanned Correllendor and the land to the south. The network funneled energy toward Nothoreadhel and was intended to help 'return' the Illendi 'homeland' of Nothoreadhel to whence it came. The effort culminated in a great ceremony called the 'making', but something was wrong. The energy did not flow evenly through the network to the homeland; instead, it flowed back out through some of the Sentinels which guided the project, and rebounded across the entire experiment. The energies were too much, and the whole effort collapsed.
In the hours it took for the energies to dissipate, the face of Correllendor changed. Nothoreadhel dropped off the face of the world into the Dreaming. The Silver Clan vanished almost entirely, and the Delwen entered the Downs for the first time. Several Avatars were propelled from the Dreaming into the waking world and embodied. Fully half of the Sentinels were destroyed, driven mad, or overrun with hostile planar creatures. Creatures all across Correllendor were warped and twisted, killed or turned into magical beasts. The shape of time was distorted.
It was a cataclysm.