Elven Kingdom of Doria
After the War of the Desert Master, massive humanoid invasions followed in Atruaghin. This spawned a time of Chaos and instability. At this moment the Tiger clan, unharmed by previous incidents made its move to take over the entire country, forcing the other clans into slavery. But, the other clans, although weakened, united against the stronger Tiger Clan. A terrible civil war began. Atruaghin himself feared the destruction of all his people. He approached Ka the Preserver with his concerns and they decided to move a large portion of the remaining Atruaghin Children into the Hollow World. To preserve the balance, members of the Tiger Clan were also brought along. In the Outer World, the civil war continued. Desperate, the clans asked for outside help against the Tigers. This call was heard by the elves. King Doriath, now no longer with a kingdom of his own, saw this as his opportunity. Brining with him thousands of elven mercenaries, many from the Red Arrow clan, he marched into Atruaghin, launching a massive campaign against the Tigers. Within a few weeks, the Tigers are all but wiped out; the remaining members are being transported into the Hollow World by Ka. (Atruaghin objected to this, but Ka remained adamant).
As payment for his aid, King Doriath asks the Clans to give him the lands surrounding the Plateau. The Clans, already being decimated, can only accept this. The Turtle clan has already completely been brought into the Hollow World. With the guidance of Ilsundal, the Trees of Life and brought back into Mystara in the new elven Homeland. (They were transported into Faerie by the Treekeepers and Ilsundal when Alfheim started to die.)
The new kingdom of Doria (Named after King Doriath) has been formed, the capital is named Elvenhome. Most Alfheim Clan elves find their way into Doria, as do surviving members of the Erewan Clan from Glantri. (Their home was destroyed by the Meteor that struck Glantri during WotI.) Even a few Sea Elves from Minrothad (Not Aquatic Elves) settle on the southern coast.
Fleshing it out:
Doria is simply a result of Doriath moving Alfheim into the Atruaghin area. The Alfheim gazetteer is very useful for fleshing out Doria's culture and clans.
Almost immediately, the elves begin expanding into the nearby Malpheggi Swamp. They begin driving out undead, lizardmen and other monsters living there. There is also, I believe, one dragon king residing in this swamp. And they begin planting trees that will absorb much of the water in the swamplands, eventually turning I into a larger forest fit as home for the elves. This is of course a long project which will take years to complete. The Swamp is officially part of Darokin, but Darokin has laid no claims on these lands and are at this time not willing to fight a war with the elves in the swamplands. Hin from the nearby Five Shires are also happy to be rid many of the monsters in the swamps that border their country. (If David Knott's Dark Shire is used, these relations could prove different and very interesting!)
On the Great Plateau, the Children of Atruaghin are entering a crisis. Even though these people have survived, their culture and way of life has been destroyed by the events over the last years. The elves aid the Children and begin training them as Foresters, similar to the Foresters of Thyatis. (In 3E terms, the Foresters of Doria should be considered Rangers.) Aiding the Children also means that the elves now will enter the Plateau. They begin planting forest here as well. One remaining Tree of Life could be brought up here.
Eventually, the elves will discover the elevator leading to the Hollow World. It is uncertain what they will do with it.
At some point, the Feadiel Clan manages to reopen the Rainbow bridge, opening a connection between Doria and the Sylvan Realm in the Far West. The Sylvan Realm is described as having very few elves, especially after the departure of the Feadiel who left probably a few years before AC1000. (During the events in CM7).
However, it is more likely that many elves from that realm had long been scattered and were able to return when Moorkroft was destroyed. Contact between the two elven kingdoms could bring increased prosperity to both. Also, it is likely that Winged Elves from Ee'aar would have found their way to the Sylvan Realm by now.
If so this is an opportunity to bring a few Ee'aar into the Known World.
This is all part of Doriath's quest for Immortality.
According to the Elves of Alfheim, Doriath has long been planning to embark on the path of the Dynast, but recent events made his prospects look bleak. Now he is back on the track, but must find time to find a Time Machine. However, establishing a Kingdom takes time, so it may be a few years until Doriath can resume his quest.
I have on purpose left details and dates vague here.
This is so that the individual GM can fiddle around with the details and fit Doria more easily into their own campaign. I suspect that Doria should at least be a blooming kingdom by 1020. Should the Net Almanac Team wish to incorporate any of this (They are more than welcome too), they might want to postpone some of the dates and events described above).