The Sacrificial Lion is a sympathetic character who is slated to die so that the audience knows that the villain is playing for keeps, or that the situation really is as dangerous and desperate as it seems. The death is ultimately unnecessary in the large scheme of things, but it does provide a shocking twist to the proceedings.
The main distinction between this and a Sacrificial Lamb is in the presentation of their characters. The Sacrificial Lamb is a throwaway minor character who is made to be likable just so that the audience feels sad when they die. Their death provides no real change to the plot, only that we know the enemy is Dead Serious.
On the other hand, the Sacrificial Lion might not be central to the plot, but the character isn't throwaway; quite often, in fact, a Sacrificial Lion will be one of the second- or even first-tier characters. Their death usually produces a noticeable shockwave to the story or changes the way the rest of the characters behave. Sometimes they die specifically to bump the villain over the Moral Event Horizon. If their death is key to the story (such as a political assassination), then it ceases to be a "sacrificial" character and becomes plot relevant.
The Evil Counterpart to this trope is the Sacrificial Vile, who is essentially a villainous and often particularly evil Lion whose death greatly impacts the nature of the story and frequently shakes up the plot.
Often found in Anyone Can Die stories. A Mauve Shirt is frequently in danger of becoming a Sacrificial Lion, if the writers decide to kill him off for a cheap shock. Decoy Protagonists can rapidly become Sacrificial Lions as well, in which case this also qualifies as a Gut Punch. This is essentially The Worf Effect taken right to the hilt. Compare the Knight of Cerebus, who probably killed him. In a video game, his death is likely to be a Player Punch.
May or may not have anything to do with actual sacrificial lions.
As a Death Trope, all Spoilers will be unmarked ahead. Beware.
Granch in Amethyst, Princess of Gemworld was an ally of the titular protagonist in the first half of the original maxi-series. As the story progresses, he rallies together his siblings for an attack on the Big Bad Dark Opal (Granch and his siblings were Dark Opal's children, but they were rejected by him due to their deformities, and Opal instead adopted Carnelian to replace them). In the end, all of Granch's siblings are killed, and Granch himself dies in battle against Dark Opal himself. This is at the half-way point of the series.
Mindfuck, from Empowered, whose death leaves Sistah Spooky barely this side of catatonic and Emp herself with a heaping dose of survivor's guilt.
Sally Avril from Untold Tales of Spider-Man, one of Peter Parker's fellow students. She tries to become a vigilante like Spider-Man, but he tries to dissuade her due to the risks. Undaunted, she later dies in an auto accident after recklessly pursuing Spidey to get photographs of him in action.
In Transformers More Than Meetsthe Eye, it doesn't become clear just how serious things have gotten at the end of season 1 until Pipes, Rewind, and Ambulon are killed in a matter of issues.
Narrowly averted and in fact exploited by the hero of all people in Story Of The Century. L reasons that even if Light and Misa had discovered that Erin was listening in on them and that Misa had gotten the chance to catch her name with her Eyes they couldn't have killed her without irreversibly incriminating themselves and having to kill everyone else afterwards to cover this up. So this Sacrificial Lion meant not only that Anyone Can Die, but that everyone would die. But only after L is killed first. Erin is NOT happy when she figures this out, thinking that L essentially used the whole task force as bait. To his credit L does set things up so that no one has to actually die...well, except Light and Watari, and of course himself.
Cedric Diggory, the former Trope Namer from Harry Potter. He's introduced a book before, given loads of Character Development, becomes Harry's friend in addition to his rival... and then is rather casually murdered, just to show how evil Voldemort is.
Mad Eye Moody in the beginning of the seventh book. Immediately after his death, the characters comment on how surprising Moody's death is given his seemingly badass and invincible nature. If Mad-Eye can die, anyone's at risk. This is especially ironic because at the time Mad Eye was riding with Mundungus Fletcher, a veritable Mauve shirt.
Hedwig. Rather distressing given that she had been Harry's faithful pet owl for the entire series up to that point. (The Film of the Book turns her death into a Heroic Sacrifice; she had escaped but came back to protect Harry.)
In Terry Pratchett's Night Watch, the main action is kicked off by Carcer's murder of Sergeant Stronginthearm. Stronginthearm was a dwarf who was leading a riot against the trolls and was drafted into the new Watch by Captain Carrot in the second of the Watch novels. Since then, he rose through the ranks, eventually becoming a mainstay of the City Watch, as evidenced by his rank. His death, while not as shocking as that of a first or second tier character, managed to effectively communicate just how dangerous Carcer was, and how the story was about to take a turn for the worse.
Another Terry Pratchett example in Men at Arms with Cuddy.
In the penultimate Skulduggery Pleasant book Last Stand of Dead Men, main character Ghastly Bespoke is killed when Erskine Ravelis revealed as the villain. Not only does it highlight how serious things have become this far into the series; it also makes it very easy to hate Ravel.
Most Redwall novels have at least one major character dying halfway through or towards the end of every book. Salamandastrontook it up a notch and killed off one of the main characters.
In Anne McCaffrey's Talent And The Hive novel, Damia, Larak Raven (younger brother and closest relative of the title character) dies when he absorbs the brunt of a psychic attack meant for another. A good amount of the book is spent establishing that Larak his coming into his own as an adult, contrasting the conflicted, unsatisfied life of his sister.
New Jedi Order pulled a pretty major one in Vector Prime, the first book of the series, with Chewbacca.
The Druid Allanon gets killed in The Wishsong of Shannara, letting you know that this really is the end of an era.
When Bigwig in Watership Down sets himself up for a toe-to-toe with Woundwort, he fully expects to go out as one of these. It's his last fight, whatever happens, and he knows it. His Crowning Moment Of Awesome is to not only subvert the trope but send Woundwort packing in fear - something that had never happened before and (given what happens shortly thereafter) never happens again.
According to Richard Adams, he'd actually intended for Bigwig to die there, but his daughter's begged him to change it (the story was originally created for them), and so he survived. The animated film adaptation, however, turns Blackavar into one of these in practically the same manner that Adams had intended for Bigwig, so it fulfills this trope.
In The Elenium, Kurik gets this treatment. After spending 3 books demonstrating how he is a match for any knight in the series, he gets unceremoniously slaughtered by Adus. Even the antagonist Martel is shocked and grieved by Kurik's death.
Adaon, son of the chief bard Taliesin, in The Black Cauldron. He had become a close friend and confidante of protagonist Taran and Taran's other friends, and his death changed Taran forever.
Later, in The High King, Coll son of Collfrewr and King Rhun of Mona also become Sacrificial Lions. Coll is particularly gut-wrenching for the reader because he helped raise Taran from infancy.
Cinna in Catching Fire. He is subjected to a No-Holds-Barred Beatdown by Capitol guards and Katniss is forced to watch but cannot help him, as she is currently travelling to the arena for her second Games. He gets killed offscreen at some point between the second and third books. President Snow did this just to screw with Katniss as she entered the arena, in the hope that her focus would be thrown off and she'd get herself killed. With Cinna's murder, character deaths were no longer restricted to the Games.
Also, Prim in Mocking Jay. Especially given that Katniss is told that Snow would never kill Prim, making him in some ways the lesser of two evils.
Mr.Henry in Fire World. He dies because he got aged by Harlans Time Rift.
Grella's fate in The Fire Ascending was a cruel, sad one. There's an entire part of the book dedicated to what happened to her.
Rudd Threetrees is an important, non-viewpoint character in The First Law trilogy, who essentially winds up as the leader to the Northmen in rebellion against Bethod. He is killed at the climax of Before They Are Hanged, forcing the Dog Man to take on that role for Last Arguments of Kings.
Every so often, promotions will give a rookie wrestler a massive push to sell him as a "young, uncannily talented phenom", with a major part of the push involving having the rookie defeat a couple well-established main eventers or former world champions.
In Flipside, Kindred is killed by Bloody Mary when attempting to subdue her without magical aid. He could have avoided her and likely knew that his chances were slim, but chose to attack her anyway to keep her from killing more civilians.
In Slightly Damned, Sakido gets sniped by a demon-slaying arrow just as the story setting moves from Hell to the mortal realm. Word of God has actually stated that Sakido's entire purpose was to have the readers grow attached to her only to see her cruelly picked off right when she, Rhea and Buwaro were about to earn their happy ending.
In Cuanta Vida, Gabry (better known simply as Red) is clever, vivacious, and determined. He's also the lover of Liam (the BLU Sniper) and the guy masterminding the entire escape plan - generally the guy most of the cast is counting on despite his quirkiness. Then, this happens.
In Homestuck, Jade's Dream Self gets killed off by Jack, mostly to show how deathly seriously broken their game is.
Jack's first action after being prototyped with Becquerel is to kill Bro, whom he had been fighting with on equal terms before.
And even later, trolls have begun to die. Including Vriska, who for a while became a patron to the comic's main character.
And even more later, Jade gets killed off to show how much Aranea is a threat to the session.
Pauline, from Our Little Adventure. Her murderer wasn't the Big Bad, but she was the first non-recoverable casualty in Julie's quest. Julie of course does not take it well, at least until Pauline's funeral which gives Julie and her friends a chance to good bye properly.
Lieutenant Bradley from Schlock Mercenary, when an attack on the tank he was flying resulted in it losing power, leaving the tank to destroy property and/or kill innocents when it finally came down unless something was done about it. Other toughs have died as well, but this guy got a lot of character development, being in the cast since pretty much the beginning of the comic, and getting a lot of screen time in the process.
In the first arc of Goblins, a lot of characters die to show that really Anyone Can Die. Then, for a long time, no major protagonist dies. Later, in book 4, a main character dies on both story arcs: K'seliss and Chief.
Virtually every single one of Sonic and Shadow's friends qualify in Super Mario Bros. Z, and it served to show just how much of a ruthless and horrific monster Mecha Sonic became, as well as the bleakness of the situation of fighting against him.
Jon, a developed but secondary character, was killed but the CRG. This utterly changed the playing board.
The death of Jet in Avatar: The Last Airbender certainly counts. He's killed soon after redeeming himself, and besides the fact that it came because he helped Aang find Appa again, his death doesn't otherwise benefit the heroes in the least (not like Yue's in the Book One finale). Add to it that he died in "Book Two: Chapter 17" - only three episodes before the (temporary) death of Aang himself - and it really emphasises the seriousness of the conflict.
Another Pixar example would be Rod "Torque" Redline from Cars 2.
Spoofed in Clone High, in which Ponce d'Leon would appear to be one of these if not for the fact that he's introduced in the same episode that he snuffs it, as well as the amusingly obvious foreshadowing of his death. Both the next-episode preview and the cold open feature the narrator making a big deal of "a clone dies tonight!"
The Incrediblesoriginally planned on having a civilian airplane pilot killed after being shot down to prove that the bad guys were playing for keeps. Fortunately for him, the creators of the film decided that it would take too long to introduce the character for long enough to make the audience care about his death, and the character's role was reduced to simply loaning Helen a plane.
Watch that scene—right after the main body of the airplane hits the water between the main characters, there's a shot looking down into the depths for a few seconds as the plane sinks. The pilot's hat was originally supposed to be drifting forlornly upwards in that shot, making it look a little pointless without it.
Master Thundering Rhino from Kung Fu Panda 2 is killed by Lord Shen's cannon.
If there was any question of just what Transformers Animated's Shockwave was made of, it was removed when he crushed Blurr into a cube.
Although it's a common subject of debate as to whether Blurr is dead or not.
Cliffjumper in Transformers Prime, who's killed by Starscream in the intro of the first episode. Surprisingly, although the set-up puts him in the category of a sacrificial lamb, his death has a significant effect on the Autobots as they come together closer as a team and with their human friends. And it makes them a little sore when confronting the Decepticons later on.
A rare villainous version occurs in Crossfire, with Breakdown getting eviscerated by Airachnid to show she's cutting ties with the Decepticons.
Don't forget Dreadwing when he tries to kill Starscream.