This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Master and Commander
The Acheron's first appearance, a ship that comes out of nowhere during an early morning fog and attacks the Surprise.
The looks on the faces of the Surprise crew during the final battle. Killick especially seems to go directly from his usual Servile Snarker persona to a Blood Knight.
A notable exception is Barrett Bonden, the coxswain. He stabs the French sailors in a workman-like fashion with barely a snarl during exertions.
Midshipman Hollom commits suicide by jumping off the side of the ship while holding a cannonball. The sight of his upturned face slowly fading out of sight as he sinks downwards is haunting.
Not to mention that he does it right in front of Blakeney, a thirteen year old boy. Certainly Nightmare Fuel for him.
And before that, all the seamen marking their faces as Hollom returns with his midshipmen mates. There's some tension building up.
The scene during a huge storm where the Surprise loses a mast, and the sailors have to cut it loose fast or the ship will broach—turn sideways to the waves and be rolled over. And there's one man still clinging to the ropes, watching his shipmates cut him loose, screaming for them to just wait for him—and his best friend is one of those taking an axe to the ropes, because he knows he has no choice.
19th century surgery on a warship. "Don't worry, it's just the laudanum talking." "It's just the laudanum talking" is Maturin's way of helping Blakeney save some face. Real men—even thirteen-year-olds having an arm amputated—don't cry. Luckily, Maturin is a genuine physician, not some ex-butcher or ex-barber with a chopper and a bucket of tar.note Another piece of Nightmare Fuel is how some "doctors" cut arms off—saw halfway through the bone, then snap through the rest and pull the arm free.
Maturin performing self-surgery. It's telling in that Padeen and Higgins (a surgeon's assistant and an apprentice surgeon) and Aubrey (a lifelong sailor who has been amongst and around wounds all his life) are all very visibly uncomfortable watching him do it.