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Nightmare Fuel: Temeraire
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 its captain to Mercy Kill it 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.
Novik reaches a new level in Blood of Tyrants. Russian ferals are not kept alive on the brink of starvation, but 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 become so savage that they actually begin to eat the wounded (human) soldiers.
The plague. Lenton has a Heroic BSOD when Obversaria dies, and after one finds out the details, one doesn't blame him in the slightest.
The bunyips in The Tongues of Serpents 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 in the sixth book. Just try to imagine scores of large, hungry, bloodthirsty sea serpents descending on a small fleet and twining and coiling around the ships. Not helped by a description of a man clinging desperately inside the mouth of a sea serpent before being devoured. It was bad enough that the ones who provoked the sea serpents to attack seemed to have been made queasy by it.
The aftermath of the Allegiance's destruction in Crucible of Gold. 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.