Nightmare

Nightmares are phantom-like apparitions of trauma created from the Darkness, often manifesting as past figures and enemies to exploit the worst fears of Guardians. First appearing in Shadowkeep, they took the form of several of The Guardian's past foes, including Crota, Fikrul and Ghaul.

Overview
Nightmares are the phantasmal creations made from the Darkness-attuned influences of the Pyramid found beneath Luna's surface by the Hive and then became active by Eris Morn upon her discovery.

Initial discovery of the Pyramid was made by the Hive that they experimented with its phantasmal essences using forbidden profane rituals, including the Cryptoglyph and the Scarlet Keep, to immunize from its trauma-inducing influence. In the absence to bring Nightmares forth themselves, the Hive opted instead to made use of the Nightmares conjured by Eris and the Guardians, taking the images of various foes and tragic phantoms they have faced in the past. The Hidden Swarm, under the command of the Daughters of Crota, are even implied having a plot to recreate the fallen Oryx as a Nightmare to renew their aggression against the Guardians; A necessary violation of the Sword Logic since the Taken King was never succeeded by his killers.

Though found across the Solar System, the Nightmares can be encountered primarily near and in the Scarlet Keep citadel.

The Nightmares are a very diverse force, taking the forms of the Hive, Cabal, Vex and Fallen, bearing all their abilities only made much stronger and more durable with unstable phantasmal essence and can even regenerate their injuries. Armor forged with said essence are the only known vulnerability against these Nightmares.

Some of the more powerful Nightmares grow so strong that they take the forms of the Guardians' most dangerous foes, each representing a certain aspect of trauma from their past and forcing them to conduct Nightmare Hunts to contain them.

Fallen

 * Nightmare Shank
 * Nightmare Dreg
 * Nightmare Wretch

Hive

 * Nightmare Thrall
 * Nightmare Acolyte
 * Swordbearer Nightmare
 * Tormented Servant Nightmare

Cabal

 * Nightmare War Beast
 * Nightmare Legionary

Scorn

 * Nightmare Stalker

Major Nightmares

 * Nightmare of Phogoth, the Untamed - Aspect of Fear
 * Nightmare of Crota, Son of Oryx - Aspect of Despair
 * Nightmare of Skolas, Kell of Kells - Aspect of Pride
 * Nightmare of Taniks, the Scarred - Aspect of Isolation
 * Nightmare of Dominus Ghaul - Aspect of Rage
 * Nightmare of Omnigul, Will of Crota - Aspect of Anguish
 * Nightmare of Zydron, Gate Lord - Aspect of Servitude
 * Nightmare of Fikrul, the Fanatic - Aspect of Insanity

Minor Nightmares

 * Nightmare of Reyiks, Actuator
 * Nightmare of Talgoth, Soul Smasher
 * Nightmare of Rizaahn, the Lost
 * Nightmare of Arguth, the Tormented
 * Nightmare of Val Sinu, Hellbringer
 * Nightmare of Lambiks-4
 * Nightmare of Volniks, Deathkeeper
 * Nightmare of Gothrax, Swarm Protector
 * Nightmare of Kelnix Reborn
 * Nightmare Servant of Ir Yût
 * Nightmare of Xortal, Sworn to Crota
 * Nightmare of Loronvis, Council Hand
 * Nightmare of Jaxx, Claw of Xivu Arath
 * Nightmare of Horkis, Fear of Mithrax