Surprisingly-Sudden Death

The protagonists are walking down a hallway or simply just relaxing and minding their own business when suddenly out of nowhere one of them is killed in a horrific manner. This often can and does fall in the category of Gorn. This form of demise can lead to The Reveal and can often lead to the confrontation with the Big Bad. Typically includes or is accompanied by Impaled with Extreme Prejudice. Sometimes is foreshadowed very briefly by the killer being shown just before the character's death. Most commonly associated with Horror and typically the responsible party is of the monster variety. It can also be used to reveal any obstacles in the protagonist's way by killing a less important character or a Red Shirt, if there are any. Not limited to villains, however, as it can also be done as part of a Mook Horror Show.

Can also be considered a subtrope of Harbinger of Impending Doom but is generally a plot twist. As in, up to this point everything has been going in the protagonists favor up until this trope is invoked. Also compare with Dynamic Entry, which is usually nonfatal and portrayed in a lighter tone. In the Back may use this. Killed Mid-Sentence is a specific sub-trope.

Anime and Manga

 * Elfen Lied is full of these, two of the most notable examples being the deaths of Kouta's sister and father: Lucy splits Kanae in half while she tearfully apologizes to Kouta, and then decapitates his father when he walks over a moment later. Both deaths happen within mere minutes of each other.
 * Hellsing: Andserson attempts this with Seras and nearly succeeds, but misses her heart.
 * Also, when Seras and Alucard are ordered to chase down the vampire couple that are massacring houses. Alucard blasts the male vampire through the door when he goes to answer it, empties a entire clip into him, reaches into his chest and crushes his heart, killing him.
 * Full Metal Panic
 * When the Arbalest makes it's first appearance in Khanka, Sousuke plays a lethal variation of hide-and-seek with five Savages. As in, every time he appears from the dark, a Savage permanently goes off the radar via being stabbed, shot or hit by a tree.
 * The entrance of Clouseau in The Second Raid. As a Savage is about to unleash some autocannon goodness on Sousuke and co. fleeing in a car, it takes a headshot from the side. Then Clouseau disengages ECS, throws one of his machetes into another Savage then expertly dodges incoming shots and stabs the third.
 * Kaname Tousen from Bleach. When fighting Hisagi, he pulls out his Hollow form, and while busy laughing at how screwed the heroes are...is promptly stabbed in the head by Hisagi before he can show anything.
 * This doesn't kill him though. As he lay dying, Tousen is filled with remorse. He asks Hisagi if he can show him his face, but is abruptly killed by Aizen, who explodes his body.

Comic Books

 * In the Left 4 Dead comic two guards just heard the alarm to evacuate their base, then they are killed by a Hunter and a Smoker before they could react.

Fanfiction

 * Rei does this in Aeon Entelechy Evangelion as a variation of her first appearance. On other hand it could been all in Shinji's head (looking at Cosmic Horrors from up close is a bad idea for your sanity) and what really happened was a Dynamic Entry by Misato's team.
 * Fallout Equestria: Steelhooves/Applesnack is on the wrong end of a Hellhound attack very early on in Chapter 39. This comes as a bit of a shock due to his status as a Canterlot ghoul, hence able to revive from most forms of damage. Turns out decapitation isn't something he can revive from.

Film

 * Alien series.
 * Alien: The dinner scene. Kane, having been infected previously leading to All There Is to Know About "The Crying Game", eats dinner after recovering appearing perfectly normal when he starts convulsing and is killed by a chest burster. And of course, in typical hungry Alien fashion, it devours the entire crew except for Ripley, even stowing away on her escape pod before being Thrown Out the Airlock.
 * Aliens: The android Bishop is apologizing for scaring the heroine. At this moment he is Impaled with Extreme Prejudice and you see the Alien Queen drop down from underneath the Dropship where it had stowed onto the ship and then rips Bishop in half with her claws.
 * Alien 3 even gets on the action. The prison warden is killed while yelling at Ripley, in a room full of other people no less!
 * Jurassic Park, this happens with the programmer played by Samuel L. Jackson when he goes to reset the circuit breakers and is eaten alive by a Velociraptor. Ellie has to run from the very same Velociraptor when she enters the bunker to reset the breaker.
 * Deep Blue Sea, Samuel L. Jackson, when he gets snarfed down by a shark as he's giving his Rousing Speech.
 * The Thing, when one of the members has an apparent heart attack and the doctor tries to revive him with a defibrillator. Turns out the team member was the titular Thing when the guy's chest splits open and literally disarms the doctor.
 * Parodied in Spaceballs (which even has John Hurt, who lampshades the entire event by muttering "Oh no, not again!")
 * The first Resident Evil movie does this not only one, not twice, not even three times but four times.
 * The first time is is when the Red Queen kills half the military team and the leader by using the lasers to kill them off one by one until the team leader avoids the deadly lasers and the Queen creates a laser net to kill him resulting in There Is No Kill Like Overkill.
 * The second time is when the remainder of the team is trying to escape and trying to open a code locked door. One of the members gets it open, and is dragged to the ground and devoured, complete with screams, to show exactly how much shit has just hit the fan.
 * The third time is after Spence backstabs the group and traps them in the lab, he tries to get on the train. He stops to inject himself with the anti-virus, and is attacked by the Hunter Licker and killed, introducing another obstacle that later tries to attack them while still trapped in the lab when the Queen unleashes a Xanatos Gambit.
 * The fourth and final time is when the Hunter Licker attacks the group as they're fleeing on the train and rips open one of the doors and snatches Kaplan and eats him outside the train, after which he attacks the rest of the group.
 * In the film version of The Lord Of The Rings: The Two Towers, this is how Hama dies. He and Gaming, riding along, minding their own business, then their horses start freaking out and then, suddenly, warg attack!
 * Serenity: The crew is being chased by a Reaver ship through a giant space battle. After some technical difficulties they manage to make a rough landing. Wash starts to repeat his signature line for the movie, congratulating himself on landing with no power to speak of, when a huge metal spike crashes through the cockpit window and takes Wash right through the heart, killing him instantly. Turns out he didn't lose the Reavers, which are still after them and the crew is still utterly screwed.
 * Used for comedy in Boondock Saints. In the middle of a relatively serious scene of the main characters getting drunk and discussing plans, Rocco slams his fists down on the table. This sets off his gun, resulting in his girlfriend's cat getting blown all over the wall to the complete shock of everyone involved.
 * Final Destination is now known for its drawn out, overly complex and infeasible death sequences but the first movie has a notoriously simple and shocking death when Terri is hit by a bus immediately after a heated argument with her friends. Cue the rewind button on a lot of home videos to check if that really did just happen.
 * A similar scene happens in The Final Destination when two characters leave a hospital deep in conversation, and one gets run over by an ambulance.
 * One death scene in Psycho was filmed to invoke this, as focusing the camera on the man's feet would clue the audience in that something was about to happen.
 * In Cache, after inviting Georges into his home, Majid slits his own throat. Apropos of nothing. In this case, it's the fact that the death is self inflicted that is shocking, as well as the apparent lack of motivation for doing so.
 * In The Departed, Billy Costigan is killed by a shot to the head instantly and with no warning whatsoever.

Literature

 * Edgar Allan Poe's Masque of the Red Death had a mysterious guest appear in Prince Prospero's Masquerade Ball glad in funeral garb and an apparent mask to mimic that of the victims of the plague the nobility had shut themselves away from. The results are not pretty.
 * Quite common in works by Derek Robinson. All part of his campaign to make sure his readers know that War Is Hell.
 * Happens with regularity in the Dale Brown novels, where human error results in the Dreamland team getting snuck up on and taking a pounding. For example:
 * In Flight of the Old Dog the Kavaznya laser cannon actually acts like a Real Life laser in being instantaneous, seemingly coming out of nowhere and effectively undodgeable to its various victims.
 * In Shadow Command EB-1C Vampires have just successfully attacked a laser cannon when Russian missiles speed in and splash them before countermeasures can be taken.
 * The Forever War, being a sci-fi war novel written by an actual Vietnam vet, takes this trope and runs with it. Causes of sudden, random death include: laser to the face, dropship crash, space suit malfunction, earthquake, surprise missile attack by your own superiors, dart to the chest, jumping too hard and leaving orbit, and slipping and falling on frozen helium.

Live Action TV

 * British sitcom Ideal: Moz mentions to Cartoonhead (a hitman) that they should "take Craig out", meaning taking him out to a club. Cartoonhead, who is on LSD at the time, misinterprets this and shoots Craig in the head.
 * Buffy the Vampire Slayer: When the Monster of the Week is introduced odds are good that it is going kill, eat or kidnap someone, possibly plural. It's a wonder anyone bothers moving to Sunnydale, since any decently run government would've labeled it a deathtrap and nuked the site from orbit just to be sure.
 * In the very first scene of the very first episode, the young guy sneaks his young female friend into the High School at night. It's dark and creepy and she's uncomfortable. It's clear he has dark and creepy [read: 'pointy teeth'] designs on her. She timidly asks if they're really all alone and when he verifies that they are, she's the one who grows teeth and puts the bite on him!!!
 * This could also apply to Tara in season 6. Warren runs into Buffy's lawn, shoots a few times with a gun, and Buffy and Tara are both hit, which sends Willow on a revenge spree.
 * In the second episode of the second season of Battlestar Galactica, "Valley of Darkness", some Viper pilots are relaxing after fighting off a Cylon basestar in the previous episode, "Scattered". Cue one of the Cylon Centurions who've boarded the Galactica looming out and slashing one of them across the chest.
 * Infamously, Paul from Dollhouse in the final episode.
 * Used several times in 1000 Ways to Die. One notorious example is the Pam Caked segment: the Alpha Bitch Pamela causes the flyer of her cheerleader group, Amber, to fall to the ground in public due to petty jealousy, and when she stands in front of the girls to gloat... she is accidentally trampled to death by the whole football team, as they enter the sports grounds through a banner that blocks her from their view.
 * Kate from NCIS dives in front of Gibbs in order to take the bullet intended for him. For a second everyone believes her dead, but she jumps right up, revealing her Bulletproof Vest. However, as they are all laughing in relief, Ari shoots her in the head and walks away.

Video Games

 * Dead Space introduces the necromorphs by having the player enter another room to shut down a security lock. A bio-hazard lock down is immediately triggered and both Red Shirts accompanying the player are ripped apart as the player watches.
 * In Doom 3, you're introduced to the Lost Souls through the horrific death of a female scientist, whose head rips itself off her body to become the monster in question.
 * In F.E.A.R. your first encounter with Alma, sorta, is after you open a gate and walk back to rejoin your squad you find that they have been skeletonized, you also get a replay of exactly what happened as well, and it's the first inkling you have that something is amiss. You spend the rest of the game being terrorized by her and almost getting killed by her.
 * In F.E.A.R. 2, after Terry Halford gives you a nice little energy weapon, it looks like you're going to be saddled with an Escort Mission...that is, until a Replica Assassin uncloaks from behind and rips the guy's head off.
 * In F.E.A.R. 3 the first real look one gets at the Creep is when it manifests in the middle of a group of Armacham soldiers and rips them apart right in front of you.
 * In Starcraft, a group of demolitions specialist marines and a Ghost have boarded a space station overrun by Zerg. In spite of the seriousness of the mission, the soldiers have brought a case of beer as they set the bomb. And, of course, as one is drinking his beer and laughing off the danger of the Zerg presence, he gets a Hydralisk talon right through the back of his skull and out his face, the first strike in the Zerg attack.
 * To be honest this is par for the course for the Zerg. Every time they show up they start off by using this trope.
 * It can be used in the game as well. It is a very good strategy to leave Zerglings or Hydralisks in an area where your enemies have to walk through and have them pop up as they're almost over them/almost passed them. If done correctly, you can kill a good chunk of an attack force before they can retaliate. Course, if they happen to have a detector...
 * inFamous: One mission called "The Informant" has a citizen telling you they saw something they shouldn't have. Now those who are at least passably Genre Savvy would come to the conclusion this is a cue for an escort mission. But instead a group of Mooks comes in and promptly shoots the citizen and you have to fight them off.
 * Inverted in Prototype, where you as Alex Mercer can turn a lethal Dynamic Entry into one of these for the victim. With the proper upgrades he can (just as an example) come screaming out of nowhere to karate kick one guy to death in a spray of gore, ride the carcass a few feet, and then flipkick the body into someone else, killing them too.
 * The tank crew actually references this trope. "Watch those hatches. We don't want any uninvited guests."
 * The ultimate example is the extremely-hard to accurately aim bulletdive. Starting from a great height, you plummet like a ballistic missile towards the ground. Given a sufficient height, the city block around where you land tends to be clear of hostiles. And anything else.
 * Solo Wing Pixy's return in Ace Combat Zero: The Belkan War is marked by shooting down PJ with his spiffy new "Morgan" superfighter's Frickin' Laser Beams.
 * In Fallout 3, just before entering GNR, Initiate Reddin gets this treatment courtesy of a Super Mutant Behemoth.
 * An E3 trailer for Call of Duty: Black Ops shows the player character moving down a tunnel, a comrade in front of him. Your comrade turns to talk to you, but a second later someone appears from Behind the Black and kills him.
 * Another one would be a guy reaching up over a railing on top of a mountain and pulling the nearby guard over the edge.
 * Modern Warfare 2 mission "Just Like Old Times" has Soap and Price abseiling from a mountain road for some Death From Above on Shadow Company mooks.
 * Prince of Persia: The Two Thrones has Speed Kills. By getting in the right spot, the Prince can ambush a mook, or sometimes two, and take him apart through Action Commands.
 * You do this in the Assassin's Creed series, all the time. While they force you to engage in open conflict from time to time, both Altaïr and Ezio are meant to be shanking people before the targets know they're there. Especially visible in the Attract Mode video for 2, where we don't see Ezio for a while until he suddenly pops up and kills the first target.
 * In Resistance 2, Hawthorne and Warner are killed by Big Bad Daedalus surprising the characters in separate instances as they walk ahead of Hale in spacious corridors by impaling them with his tentacles in order to discourage Hale from advancing to him, a sort of Final Boss Preview.
 * Subverted with Capelli, who's grabbed by the Mother Spinner and dragged off a massive tower. However, as a Half-Human Hybrid his regeneration saves him, and after the boss fight he says that his "regen kicked in just in time" and a team member jokingly comments that his video feed just made the highlights reel.
 * May appear in Sonic 06. Sonic and Elise are just walking around when BLAM! Death Ray! And hammy laughing, too.

Real Life

 * Creepy Real Life example: In the 60's, Japanese politician Inejiro Asanuma was talking on live TV. Then, a teenager named Otoya Yamaguchi pulled a Dynamic Entry on stage... and stabbed him to death.
 * Another real life example: Tommy Cooper, a famous british comedian and magician suddenly died on stage in the middle of a performance. The audience kept laughing even after curtain call thinking it was all an act until someone came out and told them he had passed away.