MarkB
Legend
The designers want all their settings to be part of the same multiverse. That's the baseline they're working from, regardless of setting - it's something they decided before they ever started working on 5e Eberron..Ugghh.
According to the official Eberron setting, the three primordial dragons − Eberron, Siberys, and Khyber − factually exist.
These three cosmic "wyrms" created a separate material plane deep inside the Ethereal Plane, where the planet Eberron now resides. Everything is created by fiat. There is no evolution. Officially.
According to the 5e Eberron setting, the elves of Eberron (!) officially originate from the polytheistic god Corellon from the Forgotten Realms. The polytheistic god Corellon factually exists in the Eberron setting.
The Eberron Drow officially originate from Forgotten Realms Lolth. And so on for Orcs etcetera. All of these races are from the polytheistic gods of the Forgotten Realms setting, officially, according to the Eberron setting.
The three dragons "recreated" duplicate versions of these races that the Forgotten Realms gods originated, so as to populate Eberron. There is no evolution. Eberron elves are artificial clones of Forgotten Realms elves, literally.
The polytheistic gods of Forgotten Realms factually exist in the Eberron setting. The races in Eberron derive from these factually-existing polytheistic gods.
Officially.
5e destroyed the Eberron setting.
Eberron Rising, Page 232.
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EBERRON AND THE MULTIVERSE
It is theoretically possible to travel between Eberron and other worlds in the multiverse by means of the Deep Ethereal or various spells designed for planar travel, but the cosmology of Eberron is specifcally designed to prevent such travel, to keep the world hidden away from the meddling of gods, celestials, and fiends from beyond.
The three progenitor wyrms worked together to form Eberron and its planes as a new cosmic system in the depths of the Ethereal Plane. They recreated the elves, orcs, dragons, and other races found throughout the multiverse and placed them in their new world, but allowed them to develop beyond the reach of Gruumsh, Corellon, Lolth, and other influences for good and ill.
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However, having started from that base position, they've bent over backwards to write Eberron's cosmology up in such a way as to have all of the rest of that multiverse have absolutely no effect on anything that happens within the setting itself.