The Abyss is believed to be a plane of existence circumscribing the absence of matter.
The Borost: Third Moldenteft expedition, led by Ruud Moldenteft, successfully performed the sorcery necessary to access the Abyss, Causeway. Upon entering the Gates of the Abyss, the members of the expedition found themselves in a completely homogeneous void, devoid of any matter or any light.
In this void was suspended the Mentor, a Borostím individual, who appeared to deliver some cryptic advice. The Mentor informed the travelers that their presence was unwelcome in the Abyss, and that any traveler without the Borost:Eyes of Manred was unwelcome in the Abyss.
After several hours traversing the Abyss, but apparently going nowhere, and with no other features in sight, the sorcery Causeway was once again cast and the Expedition returned to Borost.