Laboratory of the Gods

A megadungeon that was the foremost laboratory for the gods in the Material Plane, abandoned when the chromatic dragons forced the gods out.
The scariest thing in that megadungeon is the severed mind of the Abyss. The gods couldn't entirely kill the Abyss, so they sundered its mind from its body (similar to the Sundering of Keryth long afterwards). They hid the mind of the Abyss in the core of the Material Plane, in the deepest part of their laboratory, sealed behind the strongest seals they could make. For they all fear that if it was ever reunited with the Abyss, The Chaos From Outside would awaken once more.

RUINED STRUCTURE
Abandoned 298,000 years pre-Curse.