The Ouranucial fortress was a fortress in the sky, right below maximum build hight. It was eventually obliterated by an explosion which sent half of it's remains into the void.
The Ouranucial fortress was guarded by the Ouranucial Dragon and Iron golems guilded in a light coat of Gold. It is unknown exactly who or what built the fortress, but they are known to have long sense abandoned the Aetherical and Void fortress.
It is unknown what caused the Ouranucial fortress's explosion, but it must have been extremley powerful to destroy the 3-block-thick stonebrick walls of the fortress.