From The Heretic Knowledge Vault
This article describes the magical effect used by the elves. For a spell cast by individual mages for some of the same purposes, see Barrier.
A magical effect set up by the elves near the end of the Errant War, to allow them to wallow in self-pity without being bothered by smelly humans. What little we know of it is mainly thanks to the long monologue by Sarine in Chapter 10.
According to Sarine, the elves resorted to this step after the Errant War left them so depopulated that they had to stop fighting and retreat to their one intact city, Praenubilus Astu. It used to cover as much of the elven homeland (carved out of the southeastern part of what is now Veracia) as they could fit inside the effect. Mainly, however, it was protecting the city, since that's where the elves were. For hundreds, nearly thousands, of years, it basically worked, and elves and humans didn't interact.
However, the elves hadn't reckoned on Luminosita. Once the Veracians figured out how to control their god-construct, it was just a matter of time until they sent him to "knock on the elves' door" and try to bring down the barrier. This didn't exactly go as expected; the barrier came down, all right, but it almost caused Luminosita to become destabilized or dispelled or otherwise zapped. An unexpected side effect was that it also affected the Cloaking device concealing the long-lost city of Tsuirakushiti from the world (indeed, did some damage to the levitation systems that keep the sky city in the sky). In a nutshell, the hidden lands of the Poe-verse suddenly weren't so hidden any more, and events started careening toward the climax related by Errant Story itself. A somewhat garbled version of this event has become part of the Veracian Church history/theology, as related (rather skeptically) by Jon.
We still have very little information on how the Elven Barrier worked -- whether it needed constant tending by the elves, for example, or whether once established, it was stable until disturbed. Well, that's what sequels are for.