In the fourth layer, the narrowing tunnels finally constrict down to nothing, leaving behind an infinite number of closed-off spaces filled with stale air or vacuum surrounded by an infinitude of solid stone. The portals that connect Agathion to the rest of Pandemonium open into the otherwise unreachable bubbles, but the act of stepping through a portal always sets off a windstorm.
Unless you know where the portal is, the closed-off spaces of Agathion are almost impossible to find. For this reason, forgotten spaces have been used by deities (and other powerful entities that predate the current deities) as vaults where items are hidden away. Such items may include uncontrollable artifacts, precious mementos, lost languages, unborn cosmologies, and monsters of such cataclysmic power that they couldn't be slain or otherwise neutralized.