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, Halo 3: ODST, Halo: Reach, and Halo 4.

Legendary is the most difficult setting of Campaign gameplay in every Halo game to date, aside from the informal "Mythic" difficulty (a weekly challenge over Xbox LIVE) which refers to Legendary with all Skulls active in Halo games that feature them. The Legendary difficulty is also featured in Firefight and Skirmish games. At the end of a Halo game, there may be a cutscene that will only be available when a player has completed the campaign on Legendary.

Summary
In the original Halo trilogy and Halo: Reach, the difficulty is symbolized by an Elite skull with a bullet-hole through one side, two crossed UNSC machetes, and a shield bearing the Marathon symbol. In Halo 3: ODST it is symbolized by a Human skull with two crossed suppressed SMGs with the same shield. In Halo 4, it is represented as a shield with crossed blades and a Promethean skull.

It is regarded as a large challenge even for veteran players. Allies are practically useless aside from creating distractions. Overwhelming enemy firepower and superior numbers are plentiful on Legendary, while AI enemies have greater intelligence. One must play cautiously and conservatively to succeed. Enemies are extremely strong while you and your allies' strength is extremely low in comparison often leading the player to die frequently. Legendary on Halo 2 can be described as "Trial and Error." Completing the full Campaign on Legendary in all the Halo games unlocks an additional ending scene to that game's story. Halo: Reach and Halo 2  are the only exceptions to this.

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 but they do encounter many more enemies who are tougher to defeat and endure. Many of the same strategies that work on other settings generally function on Legendary in this game except that the player has to be more careful of one's own weaknesses than before. This is where the Plasma Pistol/Pistol combo really comes in handy. This game is arguably the hardest of the series since health packs are relatively scarce and shield regeneration is fairly slow, so the player must proceed with extreme caution, since medikits tend to be spread out.

General Differences

 * Player shields are half their Normal resistance. A few hits from Plasma will often drain them completely.
 * Enemy Shield resistance is doubled.
 * Extremely weak health. One melee from an enemy in front of you will often prove fatal.
 * Player shields and enemy shields recharge faster - twice as fast as when on Normal difficulty.
 * Enemy health is doubled.
 * Elites : Twice as many elites appear as on the Normal difficulty. Major Elites are encountered at a ratio of about one of every three elites encountered, improving AI. An area that had two Minor Elites on Normal should have four to five elites on Legendary with one or 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.
 * Enemy grunts and elites throw grenades frequently, with heavily increased speed and accuracy. The grenade detonation 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 proving deadly in combat.
 * Agility : All enemies move much faster and will rarely 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 : They are always encountered in packs of four to six with few exceptions.
 * Flood : The Flood are now armed 80 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 get stuck with grenades and most are easily killed by the enemy's increased rate of fire. Their returning fire only inflicts minimal damage on enemies. Their AI is increased significantly, still to little effect.
 * Zealots appear more frequently throughout the game. Incredibly, it takes 21 shots from a Pistol to remove their shields alone.
 * Damage is increased.

Specific Differences

 * Pillar of Autumn
 * The player does not go through the controls tutorial 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. He uses more graphic language and explains to the Marines that the Covenant will end up in puddles of their own blood. He also says, "Am I right, Marines?" with a more confident tone of voice.
 * The player has immediate access to the motion tracker and grenades.
 * The Truth and Reconciliation
 * Upon entering the loading bay, 4-6 Zealots will ambush the player and the marines in addition to the standard waves of grunts and jackals which pass through the hallways. There will also be a couple of Hunters in the hangar area where you first come up the lift.
 * 343 Guilty Spark
 * 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
 * The player only has five minutes to get to the Longsword fighter after destroying the Pillar of Autumn's engine core as opposed to the usual six minutes. The ending has also been drastically changed, now involving Johnson and an Elite.

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 extremely accurately.
 * Enemy numbers are drastically increased.
 * Shields are 50% their Normal resistance 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 resistance.
 * Virtually every elite encountered is a Major. Sometimes, The Majors will be Duel-Wielding Plasma Rifles. Major grunts are also far more common, as well as Ultras.
 * Ultra Elites are now a very common sight. You will encounter up to four in some parts of the game. They have the most powerful Shields in the game and can survive an astounding four headshots from a Sniper Rifle or Beam Rifle. They can also surprisingly survive, just barely, a Plasma Grenade. Also, Ultras and Zealots, most of the time, can survive 1 hit from a Sword, so you should think twice before approaching one of them. They can never survive, if hit in the back.
 * Elite Zealots now appear as enemies when playing as John-117. They always wield Swords and can slaughter large groups of Marines with ease.
 * Enemies are extremely adaptive and intelligent in tactical action. They rarely fall victim to being outsmarted.
 * Grunts retreat less often when a leader is killed.
 * Marine allies are much stronger. They have more health and their AI is increased significantly. They are still highly vulnerable though. They also seem to take cover very often, and are also very adaptive, and careful when engaging enemies. They also have great aim with the Sniper Rifle, and Beam Rifle, performing headshots, often.
 * Sangheili allies, now fire more accurately, and throw more grenades.
 * Unggoy Allies, now when equipped with Plasma Rifles and Needlers, fire them much more faster and accurately, like their enemy counterparts, when playing as John-117. They also throw more gernades than before.
 * Direct melee attacks received from Elites and Flood are always one hit fatalities.
 * Enemies normally have upgraded weapons. For example, Brutes that usually had Brute Plasma Rifles would now carry Carbines and also have the deadly Brute Shot equipped.
 * Enemies will often notice you sooner and prove to be more wary.
 * Jackal Snipers will appear more often than on any other difficulty. One shot from them will kill you regardless of where it hits. You should think twice before sticking yourself out into the open.
 * Stealth Elites appear more often and travel in groups of two to four, usually with one or two wielding two Plasma Rifles and the others using Energy Swords.
 * Almost all Elite Combat Forms 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 even from a Plasma Pistol will drain them completely.
 * Enemies are several times more resistant to damage. (E.g. You need three body shots or two headshots for a fully shielded Brute Captain to take it down instead of one headshot or two body shots with a sniper or beam rifle).
 * Headshots are the only means to get a quick kill on enemies like Jackals and Grunts, aside from assassinations.
 * Nearly all enemy forces are promoted by one rank with high ranking Grunts and Brutes seen more often. This 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.
 * Marine allies are severely weakened, however when given weapons they'll fire all different kinds much more rapidly with greater accuracy, similar to how the enemy acts in Normal mode.
 * 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 greater accuracy.
 * Enemies fire weapons very accurately and at an extremely high rate of fire.
 * Turrets will appear in places where there once were none, including Shades. Gunners will also fire more aggressively with a rate of fire not possible by the player.
 * Brutes will wield Brute Shots more often. They fire them faster, more accurately and they often prove deadly; a single Brute shot will kill the player. Brutes will also frequently go berserk in even a random occurrence; getting hit by the Pure Tank Form's swipe attack will kill as well.
 * The "linked lives" from Halo 2 are removed but can be achieved with the Iron Skull activated.
 * Almost every Sentinel has shields, including allied ones.
 * In singleplayer mode, the Arbiter is next to useless and will frequently be knocked out for more prolonged periods. He will also take cover a lot more and so reduce his outgoing fire however, this will also help protect him from getting knocked out as much.

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 improved with at least one or two upgrades; they also attack the player's base earlier and with more units than usual.
 * When facing Flood units, player infantry units get infected at a faster rate.
 * Extra enemy units which normally don't appear to deal with: Shades, Jackals Sniping Nests and sometimes even Mega Turrets. In Flood missions, there are more Flood Launchers, Spores and Growth Pods than usual.

Halo: Reach

 * In Halo Reach, enemy AI will be scaled according to the number of players (i.e. the more players there are, the more intelligent the enemy AI is), therefore it will not necessarily be easier to complete a mission on Legendary with more players.
 * Shade turrets fire significanty faster than physically possible by the player, and with greater sight range and accuracy. They will also notice you much sooner.
 * There is an achievement for completing the campaign alone on Legendary difficulty called "A Monument To All Your Sins." A reference to the Graveminds.
 * Elite Generals appear more often, usually wielding heavier Covenant weapons.
 * Brute Chieftains also appear slightly more often.
 * In a Bungie Weekly Update, it was stated that enemies on Legendary difficulty deal 77% more damage compared to normal difficulty. This is most likely yet another subtle seven reference.
 * Elite Minors are extremely rare on Legendary difficulty; the player will probably only see 5 or 10 of them throughout possibly the entire campaign (not counting Wraith and Banshee pilots). In their place are larger numbers of higher ranking Elites with more powerful defensive and offensive capabilities.

Halo 4

 * Promethean Knights' shields regenerate much faster.
 * Enemies deal more damage and are more resistant to damage.
 * Completing the campaign on Legendary solo will grant you the achievement Lone Wolf Legend.
 * Completing the campaign on Legendary (solo or co-op) will also give you the classic Mark VI armor and the Legendary visor color.

Trivia

 * Every game's difficulty shield icon has the Marathon symbol on it except for in Halo: Reach and Halo 4 upon close examination.
 * If The Silent Cartographer is played on any difficulty higher than 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, Halo 3: ODST, Halo: Reach, Halo: Combat Evolved Anniversary and Halo 4 keep track of whether you finished Legendary Campaign on Solo or Co-op.
 * The second Halo 3 map pack is named the Legendary map pack. (Aptly named, for the previous map pack was the Heroic map pack and the next one was Mythic map pack).
 * In Halo 3, you unlock EOD armor for beating the campaign on Legendary difficulty.
 * In Halo 3: ODST, you unlock Dare as a playable character in Firefight for beating the campaign on 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 does not have any effect when not playing on Legendary.
 * A similar style was also employed for Halo 3 DLC map packs, albeit with the difference of Battle Rifles, instead of SMGs.
 * Although the Halo: Reach difficulty logos follow the same pattern as those of Halo 3, the icon for the achievement, "A Monument To All Your Sins" (complete the campaign solo on Legendary), has a human skull instead of an Elite skull atop the shield and two combat knives.
 * The Legendary of Halo 2 is comparable to the Mythic challenge of later games.
 * Some rare dialogue can be heard during cutscenes and sometimes gameplay on Legendary difficulty.
 * Speed Running can be used to complete this difficulty in a less time consuming manner.
 * Halo 2 is considered to be the hardest Halo game to beat on Legendary difficulty.

Related Links
Campaign Complete: Legendary