Dead Serious

A bad guy cruises along for a few episodes, or sometimes a few seasons, and isn't really taken seriously. Maybe a huge war is on and you know the good guys are going to win, because of Contractual Immortality. Suddenly a main character or a fan favorite brutally dies. This shows that this threat is Dead Serious.

This can be done either to get the audience off on the "right" foot with a new villain, or to beef up a previously unimpressive one. It can also be used after a Face Heel Turn to spur the villain's former comrades into finally acting against him instead of trying to get him back on their side. Such moments also might be used for a Tonight Someone Dies rating stunt.

This trope is basically The Worf Effect pushed to its logical conclusion, a subset of Anyone Can Die, and/or one of the effects of Knight of Cerebus. If the character who is killed has a name and a well-developed background, he's a Sacrificial Lion. If he has a name, but his background is fuzzy to non-existent, he's a Sacrificial Lamb. If the dead guy has no name and no background, he's just a Red Shirt.

Compare Not-So-Harmless Villain, Stuffed Into the Fridge and Shoo Out the Clowns.

Anime

 * Hughes' death by Envy's hands from Fullmetal Alchemist.
 * In Digimon Adventure 02, when Myotismon returns as Marsh Malo Myotismon, the writers do a great job of making you really, really wanna see the bastard get what's coming to him by having him brutally slaughter his two lovable minions. Sad that after that, he proves to be all talk.
 * Krillin's death (the first time it happens!) in Dragon Ball is an example of this. It showed that Piccolo Daimou was no joke and marked the first death of a main character.
 * One Piece fans all figured that Ace would be rescued, since 'no one ever dies in One Piece' is a common mantra among the fandom. They were wrong, and shortly afterward, Whitebeard also died.
 * In Tengen Toppa Gurren Lagann, Kamina spends all his time rampaging around the landscape with his brother Simon and the Dai-Gurren Brigade before they decide to take a ridiculously huge mobile battleship as their base by stealing it. Owing to Simon freaking out over seeing a kiss between his crush Yoko Littner and Kamina, his big bro punches some sense into him; opening him up to a fatal strike from Thymilph. He's still so badass that he gets back up and avenges himself, pulling off the series' first Giga Drill Breaker, but dies immediately afterward. Cue Simon's Heroic BSOD.
 * The first sign of how utterly dark a Magical Girl series we're dealing with in Puella Magi Madoka Magica comes in the climax of the third episode, where Mami gets Eaten Alive by Charlotte.
 * Up to present day, Bleach had never killed a named character amongst the good guys - of course if we're talking about manga, and not filler/movie material. In the last arc of the manga (announced by the chapter title "Final Arc - The Thousand year war"), after the bad guys just showed up, they killed Sasakibe Chojiro, vice captain of the 1st Division. Well, his name is actually the only thing we really knew about him; but during his funeral it was established that he was actually a much stronger warrior than he looked like, and thus the new enemies are to be taken very seriously.

Comic Books

 * DC Comics have been using this trope repeatedly in their latest 'Crisis' storylines. The best example might be Dr. Light, who suffered from Villain Decay to the point that he was a joke among the DC universe, and returned to true villainy in the Identity Crisis storyline.

Film

 * Wash being killed off in Serenity, purely so that the audience would spend the rest of the battle freaking out every time a character was injured.
 * Joss jokes that he actually warned the actor, and informed said actor far in advance that this character was going to die when it went down.
 * And earlier in the same movie, Book's death, along with the destruction of every single safe haven the crew's ever had is the main impetus for the crew to actually deal with the Operative.
 * There's a story circulating that Alan Tudyk told Joss Whedon, "You should totally kill Wash at the beginning of the second season because no one will see that coming."

Literature

 * Cedric Diggory in Harry Potter.
 * As is Sirius Black himself.
 * Snape kills Dumbledore
 * The Star Wars Expanded Universe novels in the New Jedi Order series began by killing off Chewbacca in the first novel. They had actually planned to kill Luke -- but George Lucas nuked this plan.
 * More recently, Jacen Solo's murder of Mara Jade-Skywalker rounded off his Face Heel Turn.
 * Gandalf and Boromir in Lord of the Rings, facilitating the Genre Switch of the story.

Live Action TV

 * Dr Heightmeyer in Stargate Atlantis.
 * From Buffy the Vampire Slayer:
 * Up until Jenny Calendar's death, the Scoobies had been more concerned with restoring Angel's soul than actually stopping him, but killing her off apparently crossed a line that let everyone know he was serious.
 * When Warren shot Tara at the end of season six, it was the final step in showing he wasn't a villain you could just laugh off anymore.
 * It also ended his career as a villain. Willow skinned him alive in revenge the very next episode.
 * Read Season 8. Warren... kinda got better.
 * Also when Jonathan died in Season 7.
 * Edward Kerr wanted out of his contract for Sea Quest DSV season 3. The powers that be forced him to stick around for the first few episodes so as to unexpectedly kill him off midseason, underlining the show's Darker and Edgier Retool.
 * You wouldn't call the first season of Supernatural cheery, exactly, but the death of John Winchester in season two is what marks the show's sharp descent into complete misery for all major characters.

Western Animation

 * In G.I Joe Resolute Cobra Commander sets out to prove to the world that he is no longer a cowardly idiot he has Storm Shadow kill Bazooka and he blows up Moscow.
 * He also kills Major Bludd in front of his troops to show he's not taking anymore crap from them.
 * Airachnid's sudden and brutal murder of in Transformers Prime arguably qualifies, serving to illustrate that this homicidal, Axe Crazy villainess was no longer content just standing around the bridge of the Nemesis and taking orders.

Web Comics
"Hussiebot: "Oh god, you're right! There are still a few characters I haven't killed yet. I almost forgot about them.""
 * Protagonists start dying left and right in late Act 5 Act 2 in Homestuck. Interestingly, there are no fewer than 6 killers and most of them end up dying themselves or are protagonists forced to Shoot the Dog.
 * The first few times qualify, but afterwards, what with the various dreamselves, alternate timeline clones, dreambubble dwellers and what not, Killed Off for Real gets kind of hard to define. Plus, since the number of characters who haven't been killed at least once by now can be counted on one hand, it's questionable whether or not deaths are truly serious anymore.


 * Reka's death in RPG World.

Theater

 * Older Than Steam: Romeo and Juliet: Though neither side is presented as inherently more evil than the others, the death of the formally comical Mercutio as a result of the Capulet/Montague feud lets everyone know things are about to go straight to hell.