- "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, Halo 2 and Halo: Combat Evolved Anniversary.
- "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[note 1].
Legendary is the highest and most difficult setting of Campaign game play in every Halo game to date, aside from the informal "Mythic" Difficulty which is Legendary with all Skulls active in Halo games that feature them. Legendary difficulty is also featured in Firefight and Skirmish games.
 SummaryHalo Reach, the difficulty is symbolized by an Elite skull with a bullet-hole through one side, two crossed UNSC Combat Knives and a shield bearing the Marathon symbol. In Halo 3: ODST it is symbolized by a Human skull with two crossed Suppressed SMGs and the same shield. Where as in the Reclaimer Trilogy it is symbolized by a Forerunner skull, two crossed Combat Knives and a shield.
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, and AI enemies have greater intelligence.
 Halo: Combat Evolved and Halo: Anniversary
In the first Halo game, the differences between Legendary and other settings is relatively numeric. The player does not face unique or different enemies yet they face many more enemies who are tougher to defeat and endure. 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 his/her own weaknesses. This is where the Plasma Pistol/Pistol combo comes in handy.
 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 the player 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. Much faster and almost always accurately. 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 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 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.
 Specific differences
- Pillar of Autumn
- 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.
- The Truth and Reconciliation
- 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 player will get to see the non-canon secret ending.
 Halo 2
|“||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
- 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). This effect is similar to enabling the Iron Skull.
- Enemy fire or munitions have a faster velocity.
- Enemies will tend to use more powerful weapons earlier in the game or more often, with Ultras and Zealots with Energy Swords, Grunts with Fuel Rod Cannons and Needlers and finally most commonly Brutes with Brute Shots.
- Enemies will tend to stick together, and not follow the player too far from their spawn point, making it very difficult to ambush them.
- Enemies will throw grenades more often, and if the player is behind cover they may continuously throw grenades until the player moves or is killed.
- 1 burst from the Plasma Rifle is enough to kill the player (4 to penetrate shields, 3 to kill).
- 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. Major grunts are also far more common.
- Ultra Elites are now a very common sight. The player 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 head shots from a Sniper Rifle or Beam Rifle.
- Elite Zealots now appear as enemies when playing as Master Chief. 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.
- Melee attacks received from elites and Flood are practically always one hit fatalities, however moving to the side will reduce the damage concurred from such attacks.
- 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 the player sooner and prove to be more alert.
- Jackal Snipers will appear more often than on any other difficulty. One shot from them will kill the player regardless of where it hits. Players should think twice before sticking himself/herself 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.
- 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. The player needs 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.
- 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.
- Turrets will appear in places where there once were none, including Shades. Gunners will also fire more aggressively.
- Brutes will wield Brute Shots more often. They fire them faster, more accurately and they prove deadly. They will frequently go berserk in even a random occurrence.
- 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 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 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, 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 significantly faster than physically possible by the player, and with greater sight range and accuracy.
- Completing the Campaign mode on Legendary will award the Gods Must Be Strong Achievement.
- There is also an Achievement for completing the campaign alone on Legendary, called A Monument to All Your Sins.
- In a Bungie Weekly Update, a Bungie employee stated that the Legendary difficulty would be frequented by Elites with active camo and Focus Rifles, however no such enemies are present in the game.
- Elite Generals appear more often, usually wielding Covenant heavy weapons.
- Data pads 10-19 are exclusive and only found in this difficulty.
- Slight dialogue changes occur, such as Captain Keyes using Catherine Halsey's first name instead of her last when referring to her in the Pillar of Autumn level.
- Legendary is the top edition of Halo 3 and Halo: Reach.
- Every difficulty shield icon, with the exceptions of Halo: Reach, Halo: Combat Evolved Anniversary, and Halo 4, has the Marathon symbol on it.
- 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 the player have finished Legendary 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).
- There is an Achievement on Halo: Reach that requires the player to complete every campaign mission on Legendary alone.
- On Halo 3, the player unlocks EOD armor for beating Legendary difficulty.
- On Halo 3: ODST, the player unlocks Dare for beating Legendary difficulty.
- On Halo 4, the player unlocks MJOLNIR Mk. VI armor for beating Legendary difficulty.
- In Halo 2, Legendary is the only difficulty where the player can find the hidden skulls, except for the Blind Skull and the Assassins Skull. The Assassins Skull doesn't give the player the effect when not on Legendary.
- 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 a Sangheili skull atop the shield and two combat knives.
- ^ The Halo 4 description is the same wording, however it ends with an exclamation point rather than a period.