The Houses of Azath

The Azath Houses are magical prisons that were used in ages past to bind immortal spirits, such as Koschei the Deathless and the diabolical Titan, Idu Maagog (see The Legend of Idu Maagog and the Blade of Fire), who had made a pact with the Lord of the Nine Hells, in extraplanar dimensions.
No one knows how many Azath Houses were constructed, or by whom. Perhaps they were built by the gods, or perhaps by the Red Wizards or some other source of powerful sorcery.
Most are now lost and buried beneath the earth, and are thought to have been constructed by the gods themselves, sealed to mortals except for a few chosen ones known as ‘the Azath Speakers’.
The Houses are in fact mortal living entities, and one day each will perish and release its captive spirit to venture once more into the world.