There is a feminine character with nipples that are just barely depicted. If you're worried about this for any reason (ie streaming) there are mods to censor it that are easy to install on PC.
Numerous attacks from enemies involve "crushing" your character with massive objects or incredible force, but there are no visuals showing a character that has been crushed.
The Leper suffers from leprosy which cannot be cured. It is a central element of his design and character, and is shown to be something he struggles with and that people treat him differently for.
Laudnam (opium dissolved in pure alcohol) is an item used to releive Stress. The Antiquarian and Plague Doctor have abilities that involve huffing from a censer of incense and a flask of chemicals, respectively.
Heroes can become afflicted by Alcoholism (herein called the 'Tippler' quirk, and the mental illness Dipsomania, which, respectively mean they can only visit the Tavern, or are obsessed with alcohol and will go to great lengths to get drunk.)
There are several enemies in the game resembling pigs, dogs, worms, corvids, crabs, jellyfish, snails, mosquitoes (DLC), snakes (DLC), and horses (DLC).
Maggots, whelks, and carrion-eating worms are weak enemies. (Crimson Court DLC) Huge mosquitoes Half-mosquito vampires, some mutated into centipedes wearing powdered wigs and riding coats.
The Hag boss will throw one of the player party members into a giant cauldron and try to boil them alive. It's mentioned that she is a cannibal, although her eating anyone is not outright shown in the game.
Some Religeous heroes invoke holy light, burning the targeted monster. (Crimson Court DLC) The Fanatic will burn Vampirism-infected heroes at the stake.
No seizures actually happen onscreen. Though yes, if someone does get pushed to their breaking point, there is a chance that they'll get an affliction that makes them go mad.
As a precaution; the Flagellant hero class has a Crimson Court DLC trinket depicting a chipped tooth, but no action of physically damaging any teeth is shown. Implied in his comic, however.
Several Bandit enemies use whips. Flagellation (either whipping yourself or having a confessor do so) is a stress-releif ability at the Cathedral. The Flagellant Monk hero wields a scourge whip. Swine Slasher enemies target sensitive areas in combat, and they themselves have crude cybernetics replacing their hands and feet.
Skeleton enemies, and a boss who summons floating ghost heads. The Drowned Crew boss is a group of ghostly pirates and their cursed anchor. The Squiffy Ghast enemy is a ghost pirate who plays the fiddle and drinks hilarious ammounts of rum. The Narrator is the ghost of the player's uncle. A ghost not seen anywhere else in the game cameos in the intro cutscene.
There are traps that play a loud noise and quickly pan to a visual of your character dodging/taking damage, but they at most a little surprising and shouldn't worry anyone concerned about horror-game style jumpscares.
The Abomination hero lives in symbiosis with a demon. He can let it out to play nearly at-will. The Occultist channels dark forces for his abilities, and is occasionally possessed by them when Stressed Out. Swine enemies are the result of the Ancestor's demonology experiments. The Cultist and Madman enemies in the Darkest Dungeon area are at least indwelled and deformed by their uh, passengers. The mental sickness 'Demonomania,' wherein the afflicted beleives themselves to be possessed but really is not, can be develloped.
You can send your heroes to hospitals, but there's no scene that plays when you do it. Only flavor text on what's probably happening while they get treated.
Masochistic characters sometimes deliberately hurt themselves. Flagellation (beating yourself with a whip) is a stress-releif activity at the Cathedral. The Flagellant Monk Hero can do this at Camp.
There are however several lines of dialogue that afflicted heroes can say during combat about wanting to commit suicide, or otherwise begging for death.
There is a sickness called "bulimia", also some quirks (including positive ones) causes some eating disorders, eg. eating half of food portion when character is very stressed.
None of the characters are explicitly stated to be LGBT+ since the story takes place in the middle ages, though no character is openly homophobic as well.
A Class in the game is partnered with a dog, who can die if you fail to keep them alive.