Unending Nightmare is a new Atlas Keystone in PoE 3.22. With this keystone allocated, the Delirium will spawn on the map and never dissipate. Trial of the Ancestors introduces 16 new keystone passives to unlock on the Atlas Passive Tree.
Flavour Text: What if it’s all just in your head?
Unending Nightmare causes Delirium fog from the Delirium Mirror to never dissipate in your maps, preventing it from being ended by being outside of the fog; the Delirium encounter can still be ended by clicking the hotkey or button, allowing the player to claim the Delirium rewards.[confirmation needed] However, taking this notable disables the ability to find Simulacrum Splinters and Delirium Orbs as loot in your maps, either from claiming the Delirium rewards from the encounter itself, or from other loot sources.
Using this keystone nullifies the need to use The Singular Entity, but players can still choose to allocate The Singular Entity for a less intensive version of this keystone and still be able to acquire Simulacrum Splinters and Delirium Orbs from Delirium Mirror encounters and other sources.
As of version 3.22.0, it is located near the center-left of the Atlas passive skill tree inside the Delirium cluster containing The Singular Entity and Descent Into Madness.