Scars of Faerûn
Abeir-Toril is the name of a prehistoric planet that existed as one until the year -31000 DR.
It was then that the creator god Ao sundered the world into two different planet-worlds: Abeir and Toril, to end a destructive conflict between the Primordials and the Gods. Abeir was given to the Primordials and Toril to the Gods.
Toril is where the continent of Faerûn lies.
Although this separation was considered a legend by many, it was revealed to be the truth when the Spellplague caused some parts of Abeir and Toril to exchange places. Today, in Toril, west of Faerûn and the Trackless Sea, lies a continent which is called Returned Abeir.