This is to share some conjecture of my own based on information read both within these forums and in-game from varying NPCs for the purpose of clarification and discussion, specifically upon the topic of the Elezen timeline and history. This thread has the potential to spoil some aspects of 3.0. but it’s been like five months already so...
Firstly, with the coming of the expansion we were blessed with a veritable undiscovered trove of knowledge on Eorzea's history, science behind some of her many mysteries, and further elucidation on the ancestry of some of her native races with the Elezen being the focal point of the latter statement. The Elezen are a traditionally nomadic people who in former times claimed sole dominion over Eorzea, and as such developed their characteristic pride and prejudice. Now, before and after the time of the Hyurian migration I have some conjecture as to the timeline of events with the main scenario quests and all they might offer in consideration.
To begin, If we're to span back to the early conflicts twixt man and dragon (at least where the Elezen are concerned), then I would assume that the tribe of ancestral Elezen in question settled about Abalathia’s Spine in such areas as the Dravanian Forelands (and whatever regions hitherto undiscovered and/or patched in) and waged war with the horde from that front until the time of peace wherein the ancestral Elezen migrated to the mount of Sohm Al and the area of the Churning Mists. Two centuries of coexistence with the dragons (and moogles) until the war sprung anew—the Elezen living within the Churning Mists either being exodized, slaughtered or both to some extent with the survivors retreating back to mainland or perhaps another region such as Coerthas.
Trudge along a couple more centuries and we have the Hyurian migration which was akin more so to an invasion to the Elezen hubris of the era. Now, perhaps it’s simply that not all tribes of early Elezen were particularly exclusive toward Halone thus having no compulsion to migrate to the ‘promised land’ as it were and here is where I would posit the introduction of a different sect of ancestral Elezen that would be the founders of Gelmorra and Gridania therein. This sect of Elezen would instead migrate to the lavish forests of the Black Shroud, then only to realize how hostile the elementals are toward foreign and possibly threatening beings in their midst and have no choice but to retreat into the caverns for safety and seclusion, resorting to more primal and instinctive methods of survival. As time would pass, the Hyur, too, would happen upon the Black Shroud and be welcomed in much the same manner as their Elezen compatriots. For the sake of simplicity and later discussion, we shall say there was little if any warring between the Hyur and Elezen (aside from perhaps a stubborn few) of the Black Shroud, forming a unity with surviving the harsh forest and the elementals that protect it in mind.
Exactly how much time passes between then and the treaty with the elementals is unclear to me, but once formal permission for man to tread upon the overworld was given, those ties between Elezen and Hyur remained, thusly laying the foundation for what would be known as the city-state of Gridania. The stubborn few who chose to remain behind within their established cave systems thusly forming the foundation for what would come to be known as the subterranean city of Gelmorra and her inhabitants, through the process of evolution, resulting in the contemporary Duskwight.
I am eager to read responses to this thread for the sake of seeing how much of my conjecture holds true and wherein I might do more research.