Dad the Veteran: Gimli's father Glóin would qualify, being a veteran of both the Battle of Azanulbizar and the Battle of the Five Armies.
Darkest Hour: Happens at least once in each book; the Bridge of Khazad-dûm, the Breaking of the Fellowship, the Battle at Helm's Deep, the Siege of Gondor, the Battle at the Black Gates.
Dark Is Evil, Dark Is Not Evil: Different nations and peoples on both ends of the good-evil-spectrum have used black as their colour, or have black hair. It seems that Sauron likes this trope, as Éomer mentions in The Two Towers that agents of Sauron routinely steal black horses, to the point where there are practically none left in Rohan.
Dawn Attack: The Rohirrim like to do this when they're playing The Cavalry. Both Erkenbrand's charge to break the siege on Helm's Deep and Théoden's attack at the Battle of Pelennor Fields happen at dawn.
Dead Guy Junior: Or at least Departed Guy Junior. Sam names his eldest son Frodo, and a later son Bilbo. It's not known whether either of those characters is still alive at the time. Incidentally, he also names two sons after Merry and Pippin.
"He caught the glint of clear grey eyes; and then he shivered, for it came suddenly to him that it was the face of one without hope who goes in search of death."
Deconstruction: Of the more conventional heroic fantasy in which The Hero gains power to overthrow evil and achieve happiness. In Tolkien's story, anyonestrongenough to use the One Ring to defeat Sauron would themselves become, in the process, as evil as Sauron (or worsenote In one of his letters, Tolkien stated that Gandalf claiming the ring would lead to a worse situation than Sauron recovering it, because at least Sauron was openly evil, whereas Gandalf with the Ring would do evil in the name of good, thereby corrupting even the idea of good). The main protagonist is a Classical Anti-Hero who is The Only One for the job for this very reason, and his mission is to throw away the only weapon powerful enough to defeat their Enemy (several characters comment on the seeming folly of this). Moreover, at the end of the story, Frodo, rather than finding happiness, suffers from physical and spiritual wounds that will not heal, and must eventually leave Middle-earth altogether.
Although deconstruction wouldn't be around in name until a decade and a half after The Lord of the Rings was published (and then only in French), Tolkien's letters clearly show that he did intend his book to interrogate conventional ideas about heroism. Frodo Baggins's fate in particular was inspired by Tolkien's own experiences in the First World War.
Dénouement: The Scouring of the Shire is a pretty jarring denouement sequence.
Despair Event Horizon: Denethor during the Siege of Gondor, which leads to him trying to immolate himself and his son on a funeral pyre.
Despair Gambit: Sauron runs several of these, one culminating in the point immediately above. His armies also make use of it in their tactics, and the Nazgûl have it in a supernatural, weaponized form, which gets more powerful in places of terror, despair and loneliness.
Despair Speech: Denethor gives several, each more long-winded than the last.
Deus ex Machina: Eagles repeatedly show up when absolutely nobody else can get the heroes out of a situation... but never appear any other time they might be just a bit useful. Possibly a more literal situation than many, since they may have ties to the Valar.
Tolkien himself said in a letter than he realized they were a Deus Ex Machina, hence why he didn't like using them often because they make solving problems too easy. He also mentioned that the Eagles themselves are aware of being an easy Deus Ex Machina and would never stoop to being Middle-earth's taxi service.
Determinator: The Three Hunters, Gollum, Frodo and Sam, especially when going through Mordor.
Arguably the second more literally, since while the Lord of the Nazgûl was once a man (the Witch-King of Angmar, but still...) Shelob was actually the last daughter of an Eldritch Abomination that gave Morgoth a run for his money.
On the other hand, as the setting most definitively doesn't run on genetics, nobody knows which one is more eldritch.
Gandalf: That we should wish to throw him down and have no one in his place is not a thought that occurs to him.
Diplomatic Impunity: The Mouth of Sauron invokes this when it looks as if Aragorn is about to cut him up into little pieces. Perhaps surprisingly Aragorn allows it to fly, only commenting that where such laws hold diplomats are usually less insolent. It probably didn't help the Mouth too much in the long run though.
Dirty Business: When Frodo lures Gollum into the hands of Faramir's men. Gollum never forgives him even though it was for his own good, and it further cements his plans of treachery.
Disney Death: Happens to Frodo twice, first when he is stabbed by an Orc and second when he is stung by Shelob. His mithril armour saved him the first time, and Shelob used paralytic poison instead of a fatal one so she could eat him alive later.
Distant Finale: There is an epilogue with Sam and his children after having just finished a reading of the book. Tolkien decided against including it in the original LotR, but it was eventually published in HoME 9: Sauron Defeated.
Distinguished Gentleman's Pipe: In the books, pretty much everyone smokes a pipe. But the older, more established authority figures are seen smoking them more often. Especially in the films.
Divided for Publication: Tolkien hated the idea of splitting it up, but the publisher insisted on publishing the novel in three separate volumes, due to a long-running postwar paper shortage. This is the root of the mistaken belief of the Lord of the Rings as a trilogy, and ironically might have inspired the trend for fantasy to be written in trilogies.
Doomed Hometown: The Shire, though it's sort of inverted. And it gets better eventually.
Don't Think. Feel: Subverted because, after he hears that Frodo is still alive, Sam gives this admonition to himself:
You fool, he isn't dead, and your heart knew it. Don't trust your head, Samwise, it is not the best part of you. The trouble with you is that you never really had any hope.
Doorstopper: Despite being called a trilogy, it's really just one giant book. Which the publisher divided into three volumes because of its size. And, supposedly, postwar paper shortages. There wasn't enough to print a full run of the whole book, and a financial risk as success was not seen as guaranteed.
Double Agent : According to Unfinished Tales one of Bill Ferny's companions ('the squint-eyed southerner') was a spy for Saruman until the Witch-King intimidated him into working for Sauron. In the book Aragorn is shown wondering who he had been working for.
The Istari (Gandalf and co.) were explicitly told not to match themselves 'power for power' against Sauron. Their job was to inspire and enable the Free Peoples and thus they had to limit a lot of their own natural power.
Sauron left the door to Mount Doom unlocked. No matter how else fate/chance conspired against him, if there had been some sort of physical obstacle at the entrance to the Sammath Naur, the Ring could not have been destroyed.
The Dreaded: Lots of examples. The Nazgûl terrify by their nature, Sauron and the Balrog command awe and fear by reputation, and the ghosts Aragorn leads into battle terrify orcs enough that they are routed before they even know if their weapons will work.
Dreaming of Times Gone By: Both Frodo and Faramir. The latter even has the recurring dream of Atlantis' (Númenor's) fall that JRRT had in real life.
Dying Race: The Ents have lost all the Entwives, rendering them incapable of reproducing. And the Elves are slowly departing from Middle-Earth, though this is mostly because they are leaving to go to Valinor rather than dying.
Shelob, though technically she's the daughter of a full-on Eldritch Abomination and an "ordinary" Giant Spider. Though she's definitely evil, she isn't allied with Sauron — Sauron does not rule her, but he permits her to continue to dwell at the top of Morgul Pass because her presence there is useful to him.
The "nameless things" that gnaw the earth, which Gandalf and the Balrog encounter far beneath Moria, are implied to be this as well. They are supposed to be so scary that Gandalf, who didn't flinch in front of the Balrog or the Witch-King of Angmar, was too horrified to even describe them. They were supposedly intended as a subtle Shout-Out to H. P. Lovecraft's works. Tolkien's earliest writing suggests they were entities with entirely separate origin from the Ainur, and perhaps even from Eru himself, but this idea was dropped as part of aligning Middle-Earth with Tolkien's own Catholic values.
It's implied that Sauron's resurgence has awoken a number of eldritch things that were asleep. The Barrow-wights may no longer be allied with Sauron, but Tolkien's copious notes indicate that they were probably roused by the presence of the Black Riders scouting out the edges of the Shire.
Eldritch Location: Morgul Vale. And if we're talking just 'eldritch' and not 'spooky, filled with death and decay' as well, then Lothlórien and Rivendell would qualify due to the use of the Elven Rings to alter the perception and flow of time. Keeping track of time there becomes a complete Mind Screw for anyone who isn't an Elf.
Enemy Civil War: The various factions of orcs. Apparently orcs constantly killing each other when there's nothing else to fight is a side effect of Sauron filling them with rage.
Enemy to All Living Things: The Nazgûl; their horses have to be specially bred and trained just to stand being near them, let alone serving as their mounts.
Engagement Challenge: Elrond gives this to Aragorn in the backstory. If he wants Arwen's hand in marriage, he's got to earn the throne of Gondor first. After all, who wouldn't want the best possible, comfortable and safe life for their daughter? Especially since by choosing to marry Aragorn she gives up a seat on the ships to the West and her immortality.
Even Evil Has Standards: Orcs serve Sauron, slaughter each other and non-orcs in endless racist wars, eat human flesh, and torture prisoners for fun. But they do not put up with orcs who eat orc-flesh.
Evil Cannot Comprehend Good: The heroes' only hope hinges on the fact that Sauron can't even conceive of someone even wanting to destroy the Ring and give up that kind of power, let alone actually setting out to try. He just could not conceive that the creature holding the ring would not chose to be a world conqueror (See Evil Is Petty). He easily fell for Aragorn's bluff of using the Ring against him, but it never occurred to him that the real intention was to take it to Mount Doom and melt it.
The eye of Sauron is described as being quite fiery. Furthermore, Sauron's hand is burning hot, which is why the ring glows on his hand and remained red-hot when first taken from him.
The Balrogs are evil spirits of fire, burning flames cloaked in shadow and armed with flaming weapons.
Evil Is Petty: The Ring tempts its bearer with power fitted to their stature, and after it betrays Isildur, it ends up only in the hands of Hobbits and their river-folk kin, whose powers and ambitions are correspondingly small. Sméagol uses it to torment his relatives and hides for centuries at the mountains' roots. Bilbo uses the Ring to escape from his obnoxious relatives and neighbors. Frodo falls at the last minute and claims it exactly when the Ring cannot help him. And Sam's dreams of conquest are so petty (an army of enslaved gardeners?) that even he finds it ridiculous and laughs.
The net result is a Subverted Trope: Sauron expected a suitably powerful individual to claim the Ring and try to challenge him outright (at which point he could overpower or outwit them and take it back), but the very pettiness of the evil it worked (or tried to work) through its bearers meant he never got the chance.
Evil Sorcerer: Saruman, and Sauron, which, combined with the above, add up to Sorcerous Overlord. Even more so in the backstory, when Sauron was playing The Dragon to Morgoth as the sorcerer lord of the Isle of Werewolves. The Lord of the Nazgûl (AKA the Witch-King of Angmar) was also this after (and possibly before) he became a Ringwraith.
Executive Meddling: Tolkien and his publisher went back and forth about the titles of the books, with Tolkien suggesting many different possibilities for both the three published volumes and the six narrative books (including some subsequently used by Christopher Tolkien like "The Return of the Shadow" and "The Treason of Isengard", and some very literal ones such as "The Ring Goes East"). Tolkien finally settled on "The Fellowship of the Ring", "The Two Towers", and "The War of the Ring" for the volumes, and left the narrative books untitled. The publisher overruled him and went with "Return of the King" for the third volume despite Tolkien arguing that it was a spoiler for anyone picking up the series.
Exposition of Immortality: Tom Bombadil, Elrond, Treebeard and even Gollum all get in on the remembering things from long, long ago as a means of reminding or showing the reader that they're ancient beings.
Fainting: Happens to everyone, naturally, but most often to Frodo.
"Facing the Bullets" One-Liner: "Fly, you fools!". Gandalf's last words urging his companions to continue the quest before he falls into the abyss dragged by the Balrog in Moria. The trope is subverted since Gandalf does not die at that point and keeps on fighting, and even when he does die it isn't permanent, but his fellowship and the reader doesn't learn that until much later.
The Fair Folk: Not exactly — the Elves are all on the side of good — but the Rohirrim think the Lórien elves are these. For that matter, Galadriel herself isn't 100% sure she's not one. Men and Hobbits have an irritating (by Elvish standards) tendency to group elven works and Sauron's dark arts under the umbrella term of "magic". And the Wood Elves, as seen in The Hobbit, are also much closer to this than the High Elves.
Fantastic Fragility: The One Ring is a Clingy Macguffinpar excellence, except for that tiny weakness to melting via Mount Doom, though explicitly not dragon-fire, even if said dragon had power on par with the first Worms of Morgoth such as Glaurung or Ancalagon the Black. (The lesser Rings could be melted that way, and indeed four of those that went to the Dwarves suffered just such a fate.)
Witch-King: Come not between the Nazgûl and his prey! Or he will not slay thee in thy turn ... He will bear thee away to the Houses of Lamentation, beyond all darkness, where thy flesh shall be devoured, and thy shrivelled mind be left naked to the Lidless Eye.
The Mouth of Sauron says that this will happen to Frodo if the heroes do not give in to Sauron's demands.
Galadriel's temptation to take the One Ring is dramatized as a One-Winged Angel moment in the book and live-action film. Galadriel imagines herself as a Queen — "not dark, but beautiful" beyond compare. This is also literally the final test for Galadriel: by passing it her exile in Middle-earth finally ends after 7,000 years.
Sam's temptation at the pass of Cirith Ungol is dramatized in the animated ROTK. Sam imagines himself as Samwise the Strong, who would make the desert of Mordor bloom with gardens.
Andúril, Aragorn's sword and an Ancestral Weapon (in the sense that it was reforged from the shards of his ancestor Elendil's sword Narsil), is called "Flame of the West", but it never actually catches on fire, though it occasionally shines as though it were. Its a more metaphoric fire, the flame of courage and hope.
Actually, Andúril does burst into flame at one point. But that's Gandalf's doing, when he reveals that he has returned.
Follow the Leader: It started the fantasy genre as we know it, and indirectly started role playing games as we know them too. The live action movies led the way for more film adaptations based on epic fantasy books.
The wrecked and ruined plain of Dagorlad, which fills a good forty or fifty square miles outside the gates of Mordor. Tolkien gives this forsaken place one of the purplest and most horrific descriptions in the book:
Frodo looked around in horror. The gasping pools were choked with ash and crawling muds, sickly white and gray, as if the mountains had vomited the filth of their entrails upon the lands about. High mounds of crushed and powdered rock, great cones of earth fire-blasted and poison-stained, stood like an obscene graveyard in endless rows, slowly revealed in the reluctant light. They had come to the desolation that lay before Mordor: the lasting monument to the dark labour of its slaves that should endure when all their purposes were made void; a land defiled, diseased beyond all healing, unless the Great Sea should enter in and wash it with oblivion. "I feel sick," said Sam. Frodo said nothing.
The Dead Marshes and Moria are also rather harsh, and both the Old Forest and Fangorn Forest get this label all but smacked onto them.
There's also the Vale of Morgul that grows beautiful but deadly flowers, and where drinking water can drive a person to insanity. Faramir warns Frodo and Sam from drinking from any stream that flows from Imlad Morgul for this reason.
In Book I, at Bree, the hobbits hear that a lot of refugees are coming from the South. They don’t pay attention because it seems improbable the Big People will want to live in the Hobbits' small holes and houses. In Rivendell, Elrond wants to send Merry and Pippin back to the Shire because he fears dark influences there. Later in the wreck of Isengard, they find Saruman had a private stash of Longbottom Leaf. Cue Book VI, "The Scouring of the Shire", where men led by Saruman have taken over the Shire.
Sam's cousin spots a giant walking tree in the Shire, but his tale is considered unbelievable.
Frodo is unable to cast the Ring into the fireplace at Bag End when Gandalf is looking for its runes.
Friend or Foe: The fleet of the Southrons, which was supposed to reinforce the army of Sauron at the battle of the Pelennor, but was captured by Aragorn. Both the Rohirrim and Gondorians thought they were still hostile at first.