Nightmare

A Nightmare creature

Nightmares are creatures that appear as shadow-like creatures, though there seems to be two kinds of Nightmares. Some Nightmares seem to come from The Mists. On the other hand, there are Nightmares which seem to be hostile spirits, such as the ones encountered during the quest Haunted and the Vaettirs such as Nulfastu Earthbound, a fallen nature spirit that entraps other spirits (that take the shape of Vaettirs).

List of hostile NPCs

Region	Affiliation	Name	Trophy
Core
The Fissure of Woe	Fissure of Woe army (Shadow Army)4	28 (30) Abyssal 24 (26) Shadow Warrior 24 (26) Shadow Ranger 24 (26) Shadow Monk 28 (30) Shadow Beast 24 (26) Shadow Mesmer 24 (26) Shadow Elemental	Dark Remains
The Underworld	Underworld army	29 (32) Bladed Aatxe 26 (30) Grasping Darkness 18 (25) Dying Nightmare 27 (30) Charged Blackness 26 (30) Coldfire Night 26 (30) Stalking Night	Demonic Remains
Guild Wars Prophecies
Ascalon (pre-Searing)	Pre-Searing Ascalon wildlife1	3 Crypt Fiend 4 Tomb Nightmare	None
Kryta	Kryta wildlife1	15 (25) Nightmare3 20 (26) Fog Nightmare	Shadowy Remnants
Maguuma Jungle	Maguuma Jungle wildlife1	20 (26) Ravager
Shiverpeak Mountains	Shiverpeak Mountains wildlife1	22 (26) Azure Shadow	Azure Remains
Ring of Fire Island Chain	Ring of Fire Islands wildlife1	28 (30) Nightmare	Smoking Remains
Guild Wars Factions
The Jade Sea2	The Jade Sea wildlife1 (Nightmare Horde)4	24 (26) Tears of Dwayna 24 (26) Deep Nightmare	None
The Deep	The Deep army (Nightmare Horde)4	24 (26) Sapping Nightmare 24 (26) Freezing Nightmare	None
Guild Wars Nightfall
Realm of Torment	Shadow Army1	24 (26) Shadow Warrior 24 (26) Shadow Ranger 24 (26) Shadow Monk	None
		28 (30) Shadow Beast	Molten Claw
		24 (26) Shadow Mesmer 24 (26) Shadow Elemental	None
Domain of Anguish	Domain of Anguish army (Shadow Army)4	28 (30) Abyssal 28 (30) Claw of Darkness 28 (30) Wind of Darkness 28 (30) Scourge of Darkness 28 (30) Curse of Darkness 28 (30) Thought of Darkness 28 (30) Chill of Darkness	Torment Gemstone
Guild Wars Eye of the North
Depths of Tyria	Depths of Tyria wildlife1	15 (25), 20 (26) Scourge Vaettir 15 (25), 20 (26) Shadow Vaettir 15 (25), 20 (26) Mist Vaettir	Vaettir Essence
Far Shiverpeaks	Far Shiverpeaks wildlife1	15 (25), 20 (26) Scourge Vaettir 15 (25), 20 (26) Shadow Vaettir 15 (25), 20 (26) Mist Vaettir
		20 (26) Spectral Vaettir	Glacial Stone
		24 (30) Arctic Nightmare3	None
	Unknown3	20 Anguish (monk) 20 Regret (monk) 20 Sorrow (monk) 20 Anguish (necromancer) 20 Regret (necromancer) 20 Anguish (mesmer) 20 Sorrow (mesmer)	Vaettir Essence
	Underworld army1, 3	19 (29) Grasping Night 15 (25) Stalking Night 24 (30) Coldfire Night 15 (25) Stalking Night	None

1. Suspected.
2. Only encountered in The Aurios Mines.
3. Only encountered in quests.
4. The affiliations outside of parentheses are by mechanics, those found inside of parentheses are by lore.

The Guild Wars 2 Wiki has an article on Nightmare.