Legendary

"You face opponents who have never known defeat, who laugh in alien tongues at your efforts to survive. This is suicide."

- Description from Halo: Combat Evolved and Halo 2.

"Tremble as teeming hordes of invincible alien monsters punish the slightest error with instant death... again and again."

- Description from Halo 3 and Halo 3: ODST.

Legendary is the highest and most difficult setting of campaign game play on Halo: Combat Evolved, Halo 2, Halo 3, Halo Wars and Halo 3: ODST, except for the informal "Mythic Difficulty" which is Legendary with all skulls on in Halo 3 "Halo 3 ODST" and Halo 2

Summary
It is symbolized by an Elite's skull with a bullet-hole of the left side of its head, with two crossed UNSC Standard Issue Combat Knives and the shield bearing the Marathon symbol, or in Halo 3: ODST it is symbolized by a Human's skull with two Silenced SMGs and a shield. It is unclear why, as the Elites are no longer your enemies, the Legendary symbol in Halo 3 still shows an Elite with a bullet hole, perhaps Bungie thought the original was better, or didn't want to swap it out for a Brute skull. It is regarded as a challenge for even veteran players, as allies are frequently useless, except for diversions, and overwhelming enemy firepower and superior numbers often force one to play cautiously and conservatively. Enemies are extremely strong, while you and your allies' strength is extremely low, often leading the player to die frequently. Legendary on Halo 2 can be described as "Trial and Error." Completing the campaign on Legendary in all the Halo games, besides Halo 2, unlocks an additional ending to the story.

Legendary was also the name of the highest difficulty setting in Bungie's Myth series, in which it had the following flavor text: "You will brave the army of a commander who has never known defeat, and the piled dead will reach the heavens; but should you succeed, in an age not yet dawned, you will be spoken of as a god!"

Halo: Combat Evolved
In the first Halo game, the differences between Legendary and other settings is relatively numeric. The player does not face unique or different enemies, they just face more enemies who are harder to defeat. Many of the same strategies that work on other settings work on Legendary in this game, except that the player has to be extremely careful of their own weaknesses. This is where the Plasma-Pistol/Human-Pistol combo comes in handy - also frequently known as: the classic Noob Combo.

General Differences

 * Master Chief's Shields are half their "Normal" power. A few hits will drain them completely.
 * Enemy shields are double their normal strength.
 * Extremely weakened health meter. One melee from an enemy in front of you will drain six health bars.
 * Master Chief's shields and enemy's shields charge faster - twice as fast as Normal.
 * Enemy health meters are double their normal strength.
 * Elites - Twice as many Elites appear as on the "Normal" setting. Major Elites are encountered at a ratio of about one of every three Elites encountered. An area that had two Minor Elites in Normal mode will have four-five Elites in Legendary, with one-two of them being Majors. Their shields can take 28 rounds from an Assault Rifle before lowering.
 * Grunts - Twice as many Grunts appear than on the "Normal" setting. Around half of the Grunts encountered are Major Grunts.
 * Grenades - Enemy Grunts and Elites throw grenades frequently, much faster and almost always accurately. The grenades' timers are cut in half.
 * Firing Rate - All enemies fire their weapons much faster and more accurately. In particular, Jackals will fire long streams of 10-15 shots with high accuracy. Jackals will also charge their Plasma Pistols much more often.
 * Agility - All enemies move much faster and will almost never allow a player to stick them with grenades. Elites will often do their dodging maneuver immediately if hit, making it very difficult to make consistent hits on them to drain their shields.
 * Stealth Elites are always encountered in packs of four-six, with few exceptions (like on the second or third circular room of Assault on the Control Room, with a pack of Grunts, and one, always one, stealth Elite).
 * Flood are now armed 90 percent of the time. Their deadly melee attacks will drain shields by half. Many more of them wield Shotguns and Rocket Launchers than before.
 * Allies - Marines are virtually useless. They will constantly be stuck with grenades, and most are easily killed by the enemy's increased rate of fire, returning fire only inflicting minimal damage on enemies. Their AI is increased significantly, to little effect.
 * Zealots - Zealots appear more frequently throughout the game. It takes 21 shots from a pistol just to remove their shields.

Specific Differences

 * Pillar of Autumn (Level)
 * The player does not go through the controls section at the beginning of the level, the game assuming that anyone choosing the highest difficulty level has gone through it before. Instead, they will wake up and leave the cryo section immediately. This also occurs on Heroic difficulty.
 * Johnson's speech changes in the beginning of the level.
 * The player has immediate access to the motion tracker and grenades.
 * 343 Guilty Spark (Level)
 * The Shotgun is no longer available early on in the game. The only way to pick it up is to kill a Combat Form that was holding one, or from one of the dead Marines from the third room with an energy bridge.
 * The Maw (Level)
 * The player only has five minutes to get to the Longsword fighter after destroying the Pillar of Autumn's engine core, instead of the usual six minutes. You get to see the non-canon secret ending.

Halo 2
In one of the Bungie Updates, Frankie describes Halo 2's Legendary as:

''A whole new sick twist on game difficulty. Legendary includes bizarre stuff like perma-death for co-op players, meaning that you can't hopscotch like you can on other difficulty levels. Once a player dies on Legendary, both players are hurled back to the last checkpoint. It's brutal. Also, that place where you encountered two Grunts and a flowerpot on Normal? Well now they're Hunters, high-ranking, sword-carrying Elites and they're all PMS-ing. Seriously, sticking your head around a corner on level two can get it shot clean off.''

General Differences

 * Enemies fire at a much faster rate and fire far more accurately.
 * Enemy numbers are drastically increased.
 * Shields are 50% their "Normal" power and a few hits will drain them completely.
 * The Arbiter's active camouflage lasts only five seconds and takes ten seconds to recharge.
 * Enemy Shields and health are almost double their "Normal" power.
 * Virtually every Elite encountered is a Major Elite. Major Grunts are also far more common.
 * Ultra Elites are now a very common sight, you will encounter up to four in some parts. They have the most powerful shields in the game, and can take four headshots from a Sniper Rifle or Beam Rifle.
 * Elite Zealots now appear as enemies when playing as Master Chief. They always wield swords and will annihilate all your Marines with ease.
 * Enemies are extremely adaptive and intelligent in tactical action. They are very difficult to kill.
 * Grunts retreat less often when a leader is killed.
 * Marine allies are much stronger; they have more health and their AI is increased significantly.
 * Melee attacks on you from Elites and Flood are almost always one-hit kills.
 * Enemies normally have upgraded weapons. For example, Brutes that usually had Brute Plasma Rifles now usually have Carbines and also have the deadly Brute Shot equipped.
 * Enemies will often notice you sooner.
 * Sniper Jackals will appear more often than on any other difficulty. One shot from them will kill you regardless of where it hits, so you should think twice before sticking yourself out into the open.
 * Stealth Elites appear more often, and travel in groups of two-four, usually with one or two wielding dual-wielded Plasma Rifles, and the others using Energy Swords.
 * Almost all Flood Elites have shields.
 * Both players have "linked lives". That is, when one of the players dies, both are sent back to the last checkpoint regardless of whether or not only one of them died (possibly to prevent players from easily reaching the status of beating legendary on their profile, or to enhance it with the general knowledge of the above statement).
 * One shot from a Brute Shot will have the same power as a shot of the Rocket Launcher on Easy difficulty.

Halo 3 and Halo 3: ODST

 * Shields are 50% their "Normal" power and a few hits will drain them completely.
 * Enemies are several times more resistant to damage (example: You need three body shots or two headshots for a Brute Captain to take it down instead of one headshot or two body shots with a sniper or beam rifle).
 * Head shots are the only means to get a quick kill on enemies like Jackals and Grunts.
 * Nearly all enemy forces are promoted by one rank with high ranking Grunts and Brutes seen more often. This, by the way, can be changed with the effects of the Thunderstorm Skull, which promotes enemies even higher. In addition to this promotion, Brutes will not carry Deployable Covers.
 * Enemy numbers double or sometimes triple in the case of Flood encounters.
 * Enemy AI improves dramatically, enemy vehicles will be significantly harder to hijack and enemies will be nearly impossible to stick with grenades.
 * Sniper Jackals, one of the rarer enemies in Halo 3, will appear with greater frequency, though not nearly as much as in Halo 2.
 * Enemies fire much faster and toss grenades more often, and with more accuracy.
 * Enemies fire weapons very accurately and at an extremely fast rate.
 * Turrets will appear in places where there were none, including Shades, and gunners will be affected by the above skills.
 * Brutes will wield Brute Shots more often and they fire them faster, more accurately and deadlier. They will frequently go in berserker mode on any random occurrence.
 * Grunts and Jackals now shoot their weapons faster.
 * The 'linked lives' are removed, and can only be achieved with the Iron Skull.
 * Almost every Sentinel has shields, including Allied ones.
 * In Halo 3, the Arbiter is next to useless and will frequently be knocked out for prolonged periods. He will also take cover a lot more and so reduce his outgoing fire.

Halo Wars

 * Enemies' health varies from being 25% to 50% stronger than on Normal, depending on the mission.
 * Enemy units deal much more damage, up to 50% more than on Normal.
 * Enemies are always upgraded with at least one or two upgrades and attack the player's base earlier and with more units.
 * When facing Flood units, player's infantry units get infected at a faster rate.
 * Extra enemy units, which normally don't appear, to deal with: Shades, stationary Jackals and sometimes even Mega Turrets. In Flood missions, more Flood Launchers, Spores and Growth Pods.

Trivia

 * Legendary is the top edition of Halo 3.
 * If The Silent Cartographer is played on a level higher then Easy, the guitar version of "Rock Anthem for Saving the World" will play after the transmission with Echo-419 after accessing the Cartographer.
 * Halo 3 keeps track of whether you finished Legendary on Solo or Co-op, thus signifying finishing it on Solo, which is considered considerably more difficult than Co-op, which can be completed by normal players.
 * The second Halo 3 map pack is named the Legendary Map Pack (aptly named, for the previous map pack was the Heroic Map Pack).
 * On Halo 3, you unlock EOD armor for beating Legendary difficulty.
 * On Halo 3: ODST, you unlock Dare for beating Legendary difficulty.
 * In Halo 2, Legendary is the only difficulty where you can find the hidden skulls, except for the Blind Skull and the Assassins Skull. The Assassins Skull doesn't give you the effect when not on Legendary.
 * It is possible to play on an unofficial difficulty, known as Mythic; this is playing on Legendary with the Mythic Skull enabled. Some will take this further and play with all of the skulls. Mythic on Legendary will make even head shots from snipers have little effect.
 * Halo 2 is the only game in the Halo trilogy without a special ending when completed on Legendary.
 * In Halo 3: ODST, the Elite skull and the Combat Knives on the Legendary logo are replaced by two silenced SMGs and a human skull. This is to reinforce the human emphasis in ODST.
 * A similar style was also employed for Halo 3 DLC map packs, albeit with the difference of Battle Rifles, instead of SMGs.