El Cid Ploy

Give me your armor to put on my shoulders;
The Trojans might suppose I was you,
Hold back, and give the Acheans' sons a breather,
For breathing spells in war are very few.

—Patroclus, The Iliad

Whether for good or for the moment, a major leader has died or gone into Ten-Minute Retirement. He's likely to be a Supporting Leader or the Big Good / Big Bad—someone highly visible, whose presence alone could turn the tide of the battle. If he's gone, it would crush the morale of the Redshirt Army. So his team pretends he's still around, by creating the visual effect of his presence—dressing someone in his armor, finding someone that looks a bit like him, even toting his corpse around and treating it like it's alive. The ploy doesn't necessarily have to be performed for a battle—any instance of faking the image of a leader or champion to maintain order counts.

There are two twists on this that sometimes show up. Often, the person pretending to be El Cid will get himself killed, having an even more demoralizing effect. Less commonly, the fake El Cid may try to assume the real one's identity, in which case this overlaps with Dead Person Impersonation (and if he does an especially good job, Becoming the Mask).

Compare Of Corpse He's Alive, which this overlaps with if it is played for laughs. Do not confuse with Dead Person Impersonation, which involves assuming someone's identity instead of their image. Compare Emergency Impersonation.

As a Death Trope, Spoilers ahead may be unmarked. Beware.

Examples of El Cid Ploy include:

Anime and Manga

In Death Note the Kira investigation team do this to L, after Light finally manages to off him. Of course they didn't know that L had some apprentices.
In the backstory of Ooku: the Inner Chambers, the initial response to Shogun Tokeguwa Iemitsu's death from the RedFaced Pox was to hide his death until a male heir could be sired upon his bastard daughter. The deception lasted around a decade before said daughter took the reins and presented herself as Shogun Iemetsu.
Although numerous people have worn Zero's trademark outfit in Code Geass, this is played with in the finale. Lelouch originally created the masked persona Zero so that he could lead the Japanese rebels to victory over Britannia without letting either side realize he was a Britannian prince. When his rebel organization the Black Knights learned of his deception, they tried to assassinate him and then declared Zero dead to cut Lelouch off from any Japanese support. Lelouch went on to use underhanded tactics to claim the Britannian throne anyway, then essentially took over the world, ruling it with an iron fist. Suzaku, Lelouch's best friend, was declared KIA during the last major battle, as part of a ploy devised by him and Lelouch. Right before the public execution of the leaders of the Black Knights, Suzaku became the new Zero in order to publicly assassinate Lelouch, who had deliberately focused the entire world's hatred on himself so that his death would unite the many countries that had long been locked in war.
In Ghost in the Shell: Stand Alone Complex, a charismatic "hero of the revolution" of some South American country has a strange reputation for surviving assassination attempts; nearly all the people killed looked exactly like him, leading the experts to conclude that he has a ton of body-doubles. In fact, the guy did die, and his Japanese partners have been continually replicating him with imitation clones so that nobody will figure out he's dead.

Film

The Trope Namer is El Cid, the 1961 Hollywood epic about a Spanish national hero from the late eleventh century. After taking a mortal wound on the eve of a decisive battle against invading Moors at the city of Valencia, he makes his wife swear that she will make sure he rides into battle the next morning no matter what. At dawn, his body is dressed in his armor and strapped onto his horse, and as the narrator puts it, El Cid "rode out of the gates of history into legend." In reality, El Cid died nine years after conquering Valencia. The ending of the film appears to have been made up by the screenwriters.
The Akira Kurosawa movie Kagemusha is this trope gone full-time for Takeda Shingen's body double.
This would have been the ending of Terminator: Salvation if there hadn't been an Internet Backdraft when it leaked out.
The plot of Moon Over Parador is a mix of this with Prince and Pauper.
This is how A Knight's Tale gets started.
The ending of Hair.

Literature

Robert Heinlein's novel Double Star. When politician John Joseph Bonforte is kidnapped by his political opponents, a lookalike actor is chosen to stand in for him to keep his coalition together and prevent the kidnappers from winning by default.
The Prisoner of Zenda and all its adaptations, such as the movie Dave.
In Brisinger the Varden use a magically created illusion to stand in for Eragon while he's busy with dwarfish politics, to prevent the Empire from attacking them while they're riderless.
In the Young Jedi Knights book series, the Emperor Palpatine has apparently been resurrected (yet again) to lead the Second Imperium. He appears through holograms to address his subjects, but near the end of the series we find that he was Dead All Along, and was actually being HoloShopped? by his bodyguards from Return of the Jedi out of Stock Footage from when he was alive and ruling.
In Robert E. Howard's Conan the Barbarian novel "The Hour of the Dragon", King Conan is struck down by sorcery before a battle and his advisers pull an El Cid Ploy. Which fails, because the stand-in doesn't have Conan's battle experience and falls for a trap. (Conan himself asked them to strap him into his saddle so he could still lead the charge.)
The Hand of Thrawn duology has a Big Bad Triumvirate working to make it look like Grand Admiral Thrawn, who'd been dead for ten years, had come Back from the Dead. This really does combine this trope and Dead Person Impersonation.
- Hilariously enough, they shouldn't have bothered, as Thrawn's own plan for coming Back from the Dead would have started a few weeks later, and it ended up being foiled because of them.
The Lloyd Alexander novel The First Two Lives of Lukas-Kasha has La Résistance perpetuate the myth that their greatest king is alive and fighting to frighten their oppressors. In reality, his daughter is in charge, and arguably accomplishing more than her father actually did.
In Book Five of Percy Jackson and The Olympians, Silena Beauregard disguises herself as Clarisse in order to make the Ares cabin fight
- It's pretty much a direct reference to the classic example from the Iliad mentioned below, down to Silena dying in the process and Clarisse going on a Roaring Rampage of Revenge once she realises.
In the Chinese classic, Romance of the Three Kingdoms, during the final battle between the forces of Shu and the forces of Wei, Shu Prime Minister and strategic mastermind Zhuge Liang dies from pneumonia. His long-time rival, Wei's Sima Yi, is emboldened by these news, and orders a full assault. However, Zhuge Liang had known he was dying for a long time, and left plans for the contingency in the hands of his deputy, Jiang Wei. His body was propped up on a chariot, his trademark battle-fan in hand, and wheeled out at the forefront of a counterattack. Seeing Zhuge Liang take the field, Sima Yi assumed that he had yet AGAIN walked into one of Zhuge's stratagems, and immediately ordered a full retreat. The Wei army fell back in disarray, and Shu won the day. Thus, it is said that "A dead Zhuge Liang beat a living Sima Yi."
In the climactic scene in the second book of the A Song of Ice and Fire saga, the tide of battle turns with the arrival of a combined Lannister-Tyrell army headed by Renly Baratheon (who was killed earlier in the book). Turns out that he really was Garlan Tyrell, and they pulled the ploy in order to demoralize Stannis' host, which was mostly made of Renly's former allies.
- Later in the fifth book the corpse of King Cleon the Butcher is put into armor, strapped to a horse and sent out to lead the charge against the Yunkai. This looks like it might actually work against the incompetent Yunkaii commanders, until one of their sellswords charges and "kills" Cleon.
Near the beginning of the second book of the Codex Alera, the First Lord collapses from working himself nearly to death. Since word of this level of infirmity would give Lord Aquitaine and Lord Kalarus exactly the chance they're looking for to try to take over, Tavi and company have to disguise the fact that he's in a coma. They wind up breaking Max out of jail to impersonate him. Hilarity Ensues.
Happens to the eponymous protagonist at least twice in Perry Rhodan. In both cases, his disappearance leaves the post of head of state of the Solar Empire suddenly vacant, so his close friends and assistants employ doubles to fool the public and hold the Empire together. (The main problem with this kind of plot is here that said Empire is otherwise always depicted as a genuine representative democracy...only with a small clique at the top deciding to deceive the constituency about the sudden loss of their elected leader in both these cases and essentially getting away with it.)
At the end of Shadow Games, Murgen puts on the Widowmaker Armor after Croaker falls, to rally the troops at the battle of Dejagore.
Older Than Feudalism: While Achilles is off having a sulk in The Iliad, his best friend, Patroclus, pretends to be him to rally the Greeks. When Patroclus gets killed by Hector, it demoralizes the Greeks, but brings Achilles back into the fight.

Live Action TV

In Buffy the Vampire Slayer, a robot duplicate stands in for Buffy while she's temporarily dead, so that the Slayer's reputation will still scare demons away.
Gabrielle disguises herself as the temporarily-incapacitated Xena to use her reputation to try and drive off a warlord.

Oral Tradition

Rodrigo Diaz de Vivar, better known as El Cid, is the Trope Namer.
When Cú Chulainn was mortally wounded in the middle of battle, he tied himself to a standing stone so he died on his feet. His enemies still believed he was alive, until a crow landed on him. Probably not intended on his part, but still worked.

Video Games

Mega Man Zero's Copy-X is this Gone Horribly Wrong.
- Also pulled off in the second game, his Quirky Miniboss Squad (Harpuia in particular) takes control, without revealing to the populace the fact that Copy-X was dead.
- Mega Man X Command Mission has an unknown reploid taking the armor and weapons of the almost dead hero Massimo.
In Infinite Undiscovery, you play as an unassuming and cowardly young lad named Chapell... who just so happens to be a dead ringer for Sigmund, the Great Hero of the People. Sigmund's combat prowess and charisma has caused much of the world to rally behind his efforts to stop The Order Of Chains and their plans, so when he dies in battle, things look bleak. Even with most of the forces intact, without Sigmund, they'd lose the support of the kingdoms... so, of course, Chapell has to take up the mantle of Sigmund, pretending to be him at least long enough to finish the fight!
In the first Suikoden game, the leader of La Résistance, Odessa, is killed by imperials while protecting a pair of children. Your party shows up seconds later and kills the imperials. With her dying breath, she admits to being "A woman first, a rebel second", and asks you to dump her body in the nearby waterway, concealing her death so as not to demoralize La Résistance. Nobody really impersonates her, but you keep pretending that she's just off somewhere else being busy at undermining the empire, for quite a long time...
- Also alluded to in one of Suikoden V's Bad Ends: if Roy defeats the Prince in their duel, he takes his place as leader while the Prince is comatose. Roy then gets himself killed, their base captured, and their forces scattered, and there's no indication the real Prince is going to wake up anytime soon...
In Wild ARMs XF, the drifter Clarissa reluctantly pretends to be the missing-and-presumed-dead princess Alexia to lead a revolution against the corrupt government.
- with the twist that they may have switched places while very young. Thus unknowingly Alexia was actually pretending to be herself.
  - And further twisted if they turn out to have been distantly related. Yeah, it's a Mind Screw.
In Age of Empires II, you get to play the trope-naming scenario in the expansion's Spanish campaign. If the enemy reaches the immobile El Cid mounted on his horse, you lose the battle instantly.
Horrifically subverted in Jeanne D'Arc. After Jeanne plummets down a ravine and is presumed dead, the French military strongarm her childhood friend and companion Liane into posing as the Maid of Orleans, in order to bolster the army's morale and to diminish their enemies' resolve. It is at this point that the English capture her and burn her at the stake, despite her horrified screaming and pleading.
At the conclusion of Final Fantasy XII, Noah fon Rosenburg, also known as Judge Magister Gabranth, lies wounded and dying. Knowing that his young master Larsa Solidor has many enemies, even within the Archadian government and in House Solidor, he entrusts his protection to his twin brother Basch fon Rosenburg. Since Basch himself was presumed dead, he takes on the mantle of Gabranth and assumes his identity in the Archadian Empire, giving up his life as Basch to protect the tenuous peace between Dalmasca and Archades. Of course, it helps that the armor and helm of a Judge Magister are all-concealing, and the denouement shows how he cut his hair in his brother's style to further the deception.

Web Comics

Happens in Sluggy Freelance when Torg is forced to take the place of Identical Stranger Lord Torgamous de Saxones, who was at the time too sick to lead his army into battle.
Drowtales has Val'Sharess Diva'ratrika, who was secretly overthrown by her three daughters in a coup. Rather than broadcast news of her death (which would destabilize the Empire), they pretend she is still alive, and even more recently, dressing someone up as the Empress and parading her through the city. As of the 15 year timeskip at least one daughter has dropped the act and publicly declared that her mother is dead, though obviously not under the real circumstances.

Western Animation

Superman: The Animated Series, "Knight Time"; Batman goes missing (Bruce Wayne is mind controlled by Braniac) so Superman wears his costume for a while. Bane - the Bat-villain relying on enhanced strength - was not a happy camper.
In Batman: The Animated Series, this is Batgirl's origin: dressing up as Batman in an effort to add the Bat's support to Commissioner Gordon's innocence.
Smithers and Homer do this in an episode of The Simpsons, since Burns needed to make a speech himself at an investor's meeting and had almost died due to Homer's bungling. They literally string him up like a dummy, and read his speech from backstage while making his arms flail. The audience bought it completely, and suprisingly, Burns turned out to be still alive, with all the movement from being jostled around getting his internal systems jump-started. Smithers even lampshades it:

"Michael Eisner's been dead for five years, Ted Turner's just a hologram!"

Real Life

When Fidel Castro was having health problems, some people thought that he had actually died and the Cuban government had pulled one of these. He eventually went on television to deny it.
- Francisco Franco, Ronald Reagan and Kim Jong-Il were suspected of the same thing for a while.
  - This just in, Generalissimo Francisco Franco is still dead.
- There was a Russian joke that Brezhnev's slowness was a result of him being dead and replaced by remote-control dummy.
Allegedly the Scottish King Robert the Bruce was once too sick with what was incorrectly diagnosed as leprosy to lead his army against the English. His lieutenants strapped the incoherent king onto his horse anyway. The English, seeing the by-then Legendary Bruce riding against them panicked and retreated.
Paul McCartney. What?
Infamously inverted by Mel Carnahan, who was elected official senator of Missouri (with his wife serving in his place) because his name could not be removed from the ballot, despite being dead. His opponent? John Ashcroft (Attorney General during the administration of George W. Bush).
Famous Korean admiral Yi Sun-Sin was on the verge of beating the Japanese navy (for about the 23rd time) when he was struck by a single bullet. He told the few soldiers around him to fight on but not announce his death to the others until the battle was over. They did so and won.