The Curtain

The Curtain was a metaphysical barrier that separated the material world from the conceptual (spiritual) one. It prevented the spirits from flowing to our world and causing chaos, destruction and possessing people. It also prevented humans from entering the very dangerous world of the spirits.
It wasn't equally strong everywhere, as there were places where it was more or less dense. And sometimes actions of the beings of one world could change it. For example, strong emotions that would be remembered by many people could damage The Curtain (big catastrophe, serial murder, an enormous concert) and some rituals, seals, and protective charms could strengthen it locally.
In thinner places, it was possible to traverse the worlds in both ways and the broader ones prevent it, while also influencing humans' emotional state negatively.
Some of the Lords knew the way to destroy it completely, however at the start of the Extinction their number was too small to do it.
Only after they have broadcasted The Word, their numbers grew astronomically, and they could execute their plan. Even then, it would take them one full year to make the Curtain fall and death of even one of those who did it would reset their progress to zero. Happily for them, there was no one capable to stop them, and the Curtain has fallen, flooding the world with conceptual beings.

Localization

Before the Extinction, it covered all of the reality.
Now it doesn't exist anymore.

Type

Metaphysical, Astral