This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Lord of the Rings Online
The Nameless of Moria, the game's version of the "nameless things that gnaw the earth" Gandalf would not speak of further. He wasn't kidding.
The Abominations in the Dunbog and Isengard instances. Saruman has made Mutants in Middle-earth, and they are creepy-looking.
Some places of the game are designed to be Nightmare Fuel. Such places include (but are not limited to) the Barrow-downs, the Red Swamp, the Fields of Fornost, the northern half of the Trollshaws, Goblin-Town (due to the general feel of the place), most of Angmar (naturally), parts of Moria, Mirkwood, Fangorn, the Paths of the Dead, and so forth.
Volume IV: Book 4: Chapter 12 is a constant stream of nightmare fuel both in- and out-of-universe. To wit: The Rohirrim do not arrive and Gandalf loses his battle with the Witch-King, forcing your character to flee and find Pippin. The lords of Gondor are slain, including Prince Imrahil. Gothmog captures you alive and brings you to the roof of the citadel to see Denethor's death. Pippin sees this happen, and Denethor commands the hobbit to flee with his last breath. Pippin runs for the edge of the city, but is snatched away by a fell-beast. Thankfully, it is revealed at the end that all of this has merely been a foretelling woven by Denethor. Having just looked into his Palantír, the player character had been left especially susceptible to his words and unable to distinguish them from reality.note This was not a vision from the Palantír. Unlike Galadriel's Mirror, Palantíri cannot show possibilities that do not come true. To quote Gandalf's response upon the player reporting to him: "I do not think he has seen the siege itself in the Anor-stone, <name>, and that is very important to us. A palantír cannot be made to lie, though its images might deceive if the user interprets them incorrectly."
The Wastes surrounding the Black Gate are blighted and desolate, but one place stands out as especially frightening. In the north of Dagorlad, near the Dead Marshes, is the hill known as Ondoher's Folly, where the namesake king of Gondor was killed by Wainriders. The Enemy has erected a horrific temple there called Faltor-shík, the Fane of Screaming. Gondorian captives are tortured and sacrificed to the Dark Lord there, as are weak or elderlyslaves from Nurn who had been taken there after being promised freedom. It's an excellent example of just how evil Sauron and his servants are. One quest involves burning the piles of mutilated remains left to rot, to grant them some dignity and prevent further defilement. To the south is Ashtok, where Orcs pick through heaps of bones, as well as a caravan of wheeled cages bringing more slaves for sacrifice. The slaves are found to have been slaughtered in their cages after attempting to revolt. The temple is overseen by a Black Númenórean named Burudagath the Bloodletter, and the circumstances of his defeat are quite chilling. The long-dead spirits of the Wainriders have awoken. The player character takes advantage of their hatred of Númenóreans to convince them to attack Burudagath, also keeping them unaware of the Host of the West. Despite this, the player character is defeated, chained to an altar, and nearly executed by him, only to be saved by the Wainriders' arrival at the last moment. Burudagath is overwhelmed by the vengeful spirits of the captives he slaughtered (though the player cannot see them), and dies from the terror.