I don't have information about ALL of the lost cities, but for the past decade or so, I've been putting together bits and pieces of stuff I have either found in published material, or created on my own, that makes up the Trembling Plains.
The key elements of this region:
1) There are very few permanent settlements - Fort Ral, Azeth's Rest, and Dry Spring.
2) There is a predominant "race," of tribal nomads, the Eloy, who maintain a unique racial trait, in that they are all of mixed human and elven descents. Human Kurnans and Eldaarish, fleeing their northern cities when those Sorcerer-Kings began to gather annual tributes for the Dragon, ventured out onto the Trembling Plains seeking sanctuary. Here, they met with elven tribes who had once been part of the Table Land tribes, but who had also ventured onto the Trembling Plains during the Cleansing Wars, seeking escape from the 8th Champion of Rajaat - Albeorn of Brunswich (Andropinis) "Slayer of Elves.”
The two races learned the benefits of cooperation, began to intermarry, have children, and established their own unique culture. Since those days, ages ago, the Eloy have since remained an elusive and secretive group of people, endlessly roaming the Trembling Plains, migrating as the seasons change, forever weary of the sedentary races who sometimes venture into their territory.
3) The Trembling Plains provide the perfect unique ecological niche for melkillot to roam, feed, reproduce, and live in the massive herds they inherently form. This region is where the majority of "free" melkillots are captured for use in merchant trade caravans as giant beasts of burden. A possible new NPC class of melkillot hunters could be developed for those who capture and "tame" the giant beasts.
Here is the map I use to delineate the general boundaries of the Trembling Plains (notice the size comparison in contrast to the Tyr Region/Tablelands):