No dogs die, there are a few dogs in the comics who are in battle scenes but turn out fine. Demoman has one line about a dog implying that something bad might happen to them. Nothing comes of it, though.
It's been revealed through the comics that The Spy is The Scout's father, yet he doesn't know this. Most likely a riff on classic spy movies like James Bond who'll usually get with women in one movie and then never seen again. Over time however Spy has accepted Scout as his son, Scout still doesn't know this.
Demoman's mother is abusive towards him. She's shown belittling him, and even hit him on the head with her cane pretty hard in one scene in the comics. If the genders were reversed/ it was an abusive father/ Demo was a child more people would have noticed/ said something
The Spy has the ability to turn completely invisible, this allows Spy to get behind the enemy team for a surprise attack. If you play classes that get tunnel vision easily (I.E. Demoman, Heavy, Engineer, and Sniper) you are at constant risk of a Spy getting you, meaning that he could be watching you waiting for the perfect time to strike.
It is implied that two of the mercenaries fighting each other are father and son, and they can fight each other. Not really framed as domestic violence though
It's unclear what exactly Zepheniah Mann did to the Administrator, but she devotes her entire life to torturing him well past both of their natural lifespans. It's also somewhat implied that whatever he did to her is what caused her to be so evil in general.
Demoman drinks alcohol almost constantly, Spy's default model is smoking a cigarette, and Sniper is mentioned to be on pain pills to counteract the effects of a different pill that enlarged his kidneys.
In one comic, a dove is violently thrown to the floor and implied to be dead for a moment before coming back to life. A minor character--a monkey--dies offscreen in a rocketship explosion.
The Arachno-Arsonist and Terror-antula cosmetic items both feature spider imagery (the former of which turns a character into a giant spider), but are only available during Halloween or community maps that have Full Moon activated. The Creepy Crawly and Violet Vermin crates also have spiders on them.
One comic in the "Mannco No More" arc sees Soldier tied down so that someone can drill his teeth with a power drill. In the same comic, Ms. Pauling and Spy have their arms tied behind their backs.
In the Mann Vs Machine mode, getting between the tank and a solid object (e.g the wooden spike barrier in Rottenburg) kills you to prevent you from getting stuck in a wall.
Technically? There are some models that turn your character into a zombie, or disfigure you (a zipper that opens a character's head and reveals their skull, multiple cosmetics that reveal the brain, etc) but nothing is very egregious or frightening; they tend to be more cartoony
The Engineer has a prosthetic arm, though it is usually covered by rubber gloves and can only be seen if you or other players equip the Gunslinger weapon. In the comics, The Soldier's girlfriend Zhanna attempts to dislocate her thumb to escape a pair of handcuffs, and resorts to cutting the whole hand off instead.
In the comics, Spy, Scout, Demoman and Soldier are nearly hanged for their crimes until Miss Pauling speaks up and manages to convince the mayor he can't hang people.
You can hear bones break if you fall from a great height, some promotional videos involve character breaking bones or describing their bones being broken
In the comics, Soldier is tortured through having his teeth drilled in. A tooth is seen flying when Soldier hits Spy with his shovel in Meet the Soldier, and the Scout is originally missing a tooth in Meet the Medic.
In Crasher, a Halloween exclusive map, a player is turned giant to carry a giant bomb to the opponent's gate, damaging the gate and exploding the carrier.
In Bread Space, the main mission is for BLU to push a cart of explosives into a giant bread monster's mouth, exploding the monster. a giant bread monster also dies in Expiration Date.
The Demoman makes various jokes about the wives of the enemy team, some of which are sexual, but it has not been confirmed whether any of the characters are/were married. The Spy is having an affair with The Scout's mother, however, and this is stated multiple times in the game, cinematics and comics.
Some maps have ghosts that can stun the player's character if they get close. Pyro has a cosmetic that allows his head to become transparent, as well as cosmetics for ghostly figures that appear behind the classes.
Pyro and Spy have cosmetics that allow them to appear as clowns. One of the maps revolves around "Bonzo the Ancient Sumerian Circus God", who is a clown.
While The Demoman is not directly possessed, his eye socket is haunted (stated by The Medic in the comics). This leads to various eye-related mishaps during Scream Fortress.
Some lore suggests The Bombinomicon possessed The Demoman's missing eye when he was a child.
Yes, the Heavy has a taunt called the Bare-Knuckle Breakdown where he punches a mannequin repeatedly. Also worth noting the mannequin has a picture of the Scout on the head.
In a voiceline for one of the earlier updates, Miss Pauling is recorded giving an assignment to the player as Spy proves he "can teach someone to play the piano in a month using an electric collar." Miss Pauling can be heard yipping as well as electric zaps being heard.
The scout's weapon, the Boston Basher, inflicts damage to the player if they miss. Several characters rely on movement strategies that harm the player, like rocket jumping.
There are times bots will join servers, sometimes in packs and will micspam really loud and obnoxious sounds. It can get really annoying and destroy your ears.
In the comics it is heavily implied that BLU Soldier has PTSD. He barricades his front door, has multiple locks, and only answers the door while armed.
In Meet the Soldier, during a scene where the aforementioned character is running into battle, shakey cam is used for a short period. This is to simulate a character from the comic (The Director) desperately following behind to capture the footage.
There is swearing however the farthest it goes is "ass". However since this is a multiplayer game, you will likely come across some foul mouthed players.
The Spy can turn completely invisible which allows him to sneak behind enemy lines and take out people from behind. This allows players to stalk their opponents and wait for the perfect time to strike. In the community there are a group of people who get a specific loadout and will just sit in the corner of maps and watch the game unfold. These people are not dangerous and are just goofing around. There is a trolling method done by some people where they will use the Cloak and Dagger, a type of cloak the Spy can use, and say voice lines to get a reaction from the player who can't see them.
In the comics, there is a mention of non-consensual use of an implanted baboon uterus in the body of an enemy to keep him at bay, it takes place off-screen, is described graphically, and we see the baby baboon after, but they are OK. (The baboon is OK, not the man who gave birth to them. He does not die due to the birth, it is due to injuries.)
The Pyro's gender is not known by anyone, and various jokes are made regarding the uncertainty. An example of this is during the English subtitles of Meet the Pyro, where The Scout uses male pronouns but the subtitles use female ones at the same time ("He's not here, is she?")
I don't know if this counts or not, but one Line from the spy has him say "Here lies scout, he ran fast and died a virgin". I still don't know if this count since Scout isn't Asexual or Aromantic. (I'm new to the term "aphobia")
One of the Scream Fortress events involves a malevolent ancient Sumerian deity, with the tie-in comic describing instances of cannibalism and human sacrifice. Granted it’s intentionally presented in a ridiculous way since it’s contrasted with carnival imagery, but can obviously turn heads for placating to the stereotype of all non-white cultures being savages
Nothing explicit, but jokes are made by characters at each other's nationalities (e.g. "Scotland is not a real country! You are an Englishman with a dress!")
In the comics, Scout and Sniper go to a traditional (though satirical) Christian depiction of Heaven, while Medic goes to a traditional Christian Hell. However, Miss Pauling and Grey Mann seem to both go to irregular depictions of an afterlife. It’s somewhat implied that the afterlife takes on the form that the person who dies is expecting it to be, or otherwise is some sort of “ironic Hell”.
Particularly one of the Scream Fortress maps has Hell as a crucial part of it and in the comics it’s established Medic made a literal deal with the devil
Sniper walks around naked for a while in the comics after his clothes are ruined. Soldier and Zhanna strip naked and cover themselves in honey to fight.
There are voice lines that mockingly claim other players/characters are crying, (with the possible exception of Pyro) everyone who is playable in this game is a man.
Not exactly illness but a good chunk of the comic storyline is two long lived characters fighting over an element that would grant them immortality and are on death’s door
Being underwater too long will make you choke, dealing 5 damage every second coming out of the water will recover 10 health so at least stay for 2 seconds before coming out to still have all of your health when you start choking.
Enemies can be blown to bloody gibs in game, as well as zombie skins revealing organs and bones being present. Meet the Soldier shows Soldier giving a speech to a collection of severed heads, Meet the Spy has Spy's head exploding from a shotgun near the end.