This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Temeraire
SPOILERS from the previous book(s) in the series are unmarked.
The use of dragons in the Napoleonic Wars. Imagine what it would be like to be on a ship that's destroyed at the hands of a dragon, particularly one that spits acid.
Heck, the spitting acid is disturbing enough. A random french dragon gets hit in the face with a lot of acid towards the end of His Majesty's Dragon, forcing her captain to Mercy Kill her before things can get any nastier. And one of Lily's harness-men dies in Empire of Ivory by falling from a cliff after acid in a handhold destroys his hand.
Traditional dragonfire is no treat either, but at least you tend to die horribly faster.
Ground combat can be leagues more horrifying; as dragons abscond with entire cavalry units, and gouge bloody furrows through rifle companies.
The sheer number of times dragon captains or crew fall to their deaths. Occupational hazard yes, but not any less awful for it. Some choice examples:
An unamed French captain mercy kills his (probably) fatally injured dragon. While flying. The narration notes that he would have been able to jump to safety, but gave up his chance.
Poor Digby jumps for an badly-secured egg to keep it from falling off Temeraire. He does catch it, but in midair, far off the ground.
Empire of Ivory
The plague. Lenton has a Heroic B.S.O.D. when Obversaria dies, and after one finds out the details, one doesn't blame him in the slightest.
Tongues of Serpents
The bunyips live in the Australian desert, lurking underground beneath special pit traps and strike like giant, reptilian trapdoor spiders, snatching people away before they even have time to scream and often so fast that no one around them even notices.
The fact that they are intelligent enough to communicate between lairs and prepare traps for dragons too big to attack directly add to the fuel. YMMV on if their ability to comprehend and willingness to accept bribes mitigates things or adds even more to it.
The attack of the sea serpents. Just try to imagine scores of large, hungry, bloodthirsty sea serpents descending on a small fleet and twining and coiling around the ships. It was so grisly that the ones who provoked the sea serpents to attack aided in rescue efforts.
Laurence could see directly down into its jaws and throat... a pallid hand within desperately clinging to the tissue of the gullet, a face bloodied but not yet senseless gazing up at him in utter horror...
Crucible of Gold
The aftermath of the Allegiance's destruction. Three dragons flying for three days straight over the open ocean with next to no rest, 200 or so passengers between them, and no water is at best an extended Oh, Crap! moment for the reader. And at least as many people dead by either the explosion or exposure to the freezing water of the ocean.
Blood of Tyrants
Russian ferals are kept on the brink of starvation, and their wings are kept chained so they can't even fly away. A clever French officer gets the idea of turning the starving ferals into a weapon by unchaining them and setting them loose. The dragons are so savage that they actually begin to eat the wounded (human) soldiers - something no sane dragon has been seen to do in the series.
League of Dragons
Laurence has some very unpleasant nightmares after recovering from a bullet wound from a duel gone awry. The last one we're treated to is especially nasty: Laurence stumbling upon Temeraire's corpse being devoured by a hungry feral to a backdrop of a stark white Russian winter.
Almost immediately after the above, Temeraire almost gets burned alive by peasants hoping to cashing on the Tsar's reward for killing a feral dragon. Temeraire was mistaken for one for travelling alone and without his harness on. He even develops post-traumatic stress from the ordeal, so much so that watching a pavillion catch fire is enough to make him recoil.