Abeir-Toril

Abeir-Toril was the name of a prehistoric planet that existed before  when Ao sundered the world into two worlds: Abeir and Toril. The name was archaic, meaning "cradle of life".

Geography
The shape of the planet was very different from either modern Abeir or modern Toril, consisting of one massive supercontinent known as Merrouroboros.

Inhabitants
Abeir-Toril was dominated by abominations from the Far Realm, among them the most prominent were the aboleths from the Abolethic Sovereignty. Other races that lived in this prehistoric world were the giantkin and the mysterious Creator races.

History
Abeir-Toril was one of many worlds that appeared after Ao crafted Realmspace out of the phlogiston. Though barren and devoid of life, creatures of manifest entropy and elemental nature known as the primordials began to appear in Realmspace, coveting those worlds. Unknown to the primordials, the first gods also began to appear in Realmspace. Selûne and Shar were the first ones, and they began to create life on the world of Abeir-Toril, creating the goddess Chauntea and the sun to sustain that life.

When the twin goddesses became aware of the threat of the primordials, they rallied all the gods to defend the virgin worlds, even summoning gods from other dimensions to aid the native gods in the war. This conflict was later known as the Dawn War. While the gods and primordials battled for ages, life flourished on Abeir-Toril.

The first lifeforms on Abeir-Toril were aberrant, however. Beings from the Far Realm who tried to gain a foothold in Realmspace, among them the Eldest, the oldest-known aboleth and possibly the progenitor of the entire species. The gods also opposed against them, and the Cerulean Sign was a rune of power created during this time by the gods to oppose those beings.

It was the end of the Shadow Epoch, during the last period of the Days of Thunder, that a conflict between the titans and the batrachi sparked. To defeat their enemies, the batrachi summoned many primordials from their imprisonment. The gods quickly moved to battle their ancient foes, and a primordial called "Asgoroth the World Shaper" hurled an ice moon down to the surface of Merrouroboros in an event known as the Tearfall, intending to destroy the world she could not claim as their own.

It was this action that finally spurred Ao to action, leading to the "changing of stars", in which the world was separated into two twin planets, Abeir and Toril. Ao gave the primordials control over Abeir, while retaining sovereignty over Toril for the gods. When Abeir-Toril was sundered, the city of Xxiphu, the realm of the aboleth, remained on Abeir.

Those events were unknown to Faerûnian sages until after the Spellplague, when the dragonborn of Abeir and the sarrukhs of Okoth revealed forgotten lore regarding the ancient past.