In the Horgon Era, the Nermais Sea was a natural and seemingly impenetrable barrier between the fire giant lands to the north under the Hofthorm Monarchy and the frost giant lands on the southern side under Glangveif. When Hofthorm started building ships at Silnir, they turned to raiding the gold-rich settlements of Glangveif. This grew worse with time leading to many wars between these two Titan Empires.
In the First Epoch, with the Titan Empires in decline, the sea was home to many smaller peoples. The most dominant came to be three halfling city-states known as the Nermanis Syndicate.
In 920, the waters of this great sea began to recede. A highly religious people, many Nermaneans thought they had done something wrong, been forsaken by Avandra. The real cause ended up being the Mîmêk city-state Sigatharkin and their war with Runelderil. This was the Onyx Enmity War (915 - 920), when in the last year of the war, Sigatharkin engineers collapsed part of Zarag-Gûn. This opened a hole in the seabed of the Nermanis Sea.
The drow account of this cataclysmic disaster says Sigatharkin's esteemed Mîmêk, in all their skill, failed to realize that the cavern ceiling was also a seabed. The Mîmêk account says the engineers miscalculated the charge and strength of the bedrock.
When the demolition went off, a catastrophe ensued. The force of incoming water expanded the hole, drowning many on each side, entire units disappeared. It devastated the lands beneath the surface, and then those on the surface. It led to famine and drought, eventually bringing down the Nermanis Syndicate (712 - 923).
The Nermanis Sea had long kept the otherwise arid region around in equilibrium, with frequent rainfalls and cool breezes. The loss of this sea to the blackness below changed these conditions for centuries. Another affected by the draining were the kuo-toa of Gholompotl. They saw their city, once submerged near the sea's bottom, be exposed to the "dryness and blistering rays of the stars", forcing abandonment.
Beneath, in the subterranean region Faelgor, the draining sea submerged Zarag-Gûn and parts of the Sigan-Kûn passage. The subterranean cities Ilaleztice and Sigatharkin were submerged for many decades. Sigatharkin's survivors went north founding the settlement Nurum-Maluk, while those of Ilaleztice joined other Runelderil holds.
Two centuries after the Nermanis flood, the climate of central Cinazan stabilized. It had become more temperate, verdant in areas the sea left behind. The old seabed became a rich fertile basin that today fulfills the grain requirements of the Khazarkar Empire and some.