"They let me pick, did I ever tell you that? Choose whichever Spartan I wanted. You know me. I did my research. Watched as you became the soldier we needed you to be. Like the others, you were strong and swift and brave. A natural leader. But you had something they didn't. Something no one saw... but me. Can you guess? Luck. Was I wrong?"—Cortana
Picking up directly from Halo 2, Master Chief returns to fight for Earth and discover the secrets of the Halo builders. This game earned $170 million in its first day, breaking all previous records (including Halo 2's).
The campaign was no longer split into two parts, instead having a single co-op campaign revolving around the Master Chief and the Arbiter working side by side. The story wrapped up the trilogy and shows the conclusion of the war.
The gameplay was kept largely intact and identical to the previous games, only refining the changes made in Halo 2. The only real new features are the weapon types (a complete subset of Brute weapons and two additional grenade types to the standard frag and plasma, spike and incendiary) and equipment you can deploy that changes the battle dynamics (impenetrable "bubble shield", trip mine, radar jammer, etc).
- After the End: This game kicks off just as the Covenant have killed nearly all of humanity, with the Flood not far behind.
- Alien Sky: Once you get to The Ark, the sky features an entire galaxy. Go ahead. Take a moment to stare at it.
- Arc Words: "Were it so easy."
- Back-to-Back Badasses: Master Chief and Arbiter pull this near the end of the game.
- Badass Boast:
Elite: "Brute ships, staggered line! Ship Master! They outnumber us three-to-one!"
R'Tas Vadum: "Then it is an even fight."
- Bash Brothers: The Chief and Arbiter are paired off with one another in single player and for the entirety of co-op.
- Big Damn Heroes: Twice. First during the Flood invasion, the second in the finale against Guilty Spark.
- Bittersweet Ending: Truth is killed, the Flood are destroyed on Halo, and the War is over, but Master Chief and Cortana are lost in space at the end of the Forward Unto Dawn with no way to get home, and the Chief is forced to go into cryogenic stasis to stay alive.
- Bond One-Liner: During Truth's death:
The Prophet of Truth: I AM TRUTH! The voice of the COVENANT!
Arbiter: And so, you must be silenced.
- Book Ends:
- "Were it so easy." The first time, before the first level begins, it's said by the Arbiter as a response to Sergeant Johnson telling him and the Master Chief not to kill each other. The second time, at the end of the game, it's said by the Arbiter again, as a response to Lord Hood's statement: "It's hard to believe [the Master Chief] is dead." And I'm not sure which would be worse, the what-if idea that the Chief had died, or the fact that the Chief is still alive, and yet no one back on Earth knows any better.
- Halo: Combat Evolved opened with Master Chief being thawed from cryo-sleep. Halo 3 ends with him going back into cryo-sleep.
- Call Back: Escaping the exploding station in a Warthog.
- Catastrophic Countdown: a variant. After starting up the Halo Ring, it needs several minutes to charge and literally shakes itself apart. The last section of the game is a mad dash to find a way off. While there's no explicit timer, the structural plates fall off at a constant rate. Justified in that the thing was half-built when it was fired.
- Charged Attack:
- The Spartan Laser takes about three seconds to fire, but destroys everything that it touches (and has sniper-grade range) and goes through your units, too (although a tank and scenery can stop it).
- The Plasma Pistol, which can now stun vehicles.
- Co-Op Multiplayer: Not only was Halo 3 one of the first console games to include online co-op, but it also had a campaign mode where players could earn points from killing enemies and compete against each other to earn the most. Basically, Competitive Cooperative Multi-player.
- This is a zigzagged case of Gameplay and Story Segregation. In 1P, the Arbiter is a friendly NPC who is present in almost all levels. In two-player co-op, he's the second player's avatar. In three-or-more player co-op, two more Elites just magically appear out of nowhere, with no particular plot justification, relevance or lines. They actually do have canon personalities (though not anything elaborated on in-game). Basically, one is a hardened veteran who is rather suspicious of humans, and the other is a young rookie who might as well be an otaku for us.
- Colossus Climb: This is generally how Scarab fights play out.
- Continuity Nod: Nearly everything Cortana says in her cryptic remarks during the game are near exact quotes of lines in The Fall of Reach, which was the jumping point for the entire Expanded Universe.
- Cyber Cyclops: Brute Stalkers, the Replacement Mooks for the Stealth Elites from the first two games, have this look going on. Though you probably wouldn't notice, due to the fact that (a) they're invisible most of the time you fight them, and (b) they only appear in two places throughout the entire campaign. Bonus points though for the eye itself being red.
- Damn You, Muscle Memory!: Many people in this game's beta ended blowing themselves up with a mine after attempting to reload their weapons, after being used for so long with pressing X. In more hilarious cases, they deployed mines below Warthogs.
- Darker and Edgier: The first two games largely avoided the implications of just how many had died/were dying in the Covenant War, but this game has Earth After the End and takes an Anyone Can Die approach to the main characters.
- Difficult but Awesome: The Spike grenades appear to be weaker versions of the plasma grenades, until you learn that their explosive pattern is conical from the place of impact. They're meant to be stuck on walls and around corners you can fill a hallway with deadly spikes that can kill from upwards of 40 feet away.
- Dual Boss: The two Scarabs in The Covenant.
- Duel to the Death: Brute society relishes in the chances to defend their honor in duels to the death. In the level The Ark, a Brute Chieftain challenges the Master Chief to have one with him, with even the formality of having all his Mooks standing in a semi-circle to watch you two fight. Guess who wins.
- Enemy Mine: The humans and Separatist Elites and temporarily the Flood against the Covenant Loyalists.
- Foreshadowing: There are multiple hints towards the rebuilt first Halo, including Guilty Spark's curious "Oh my!" when first accessing the Ark's computer systems and the control room showing ring 4 as incomplete (but not utterly destroyed, as it was after Halo: Combat Evolved).
- A God Am I:
- "My feet tread the path... I shall become a God!"
- The Gravemind apparently had delusions of godhood as well. Though they might not actually be delusions with him.
- I Like Those Odds:
Elite: "Shipmaster, Brute Ships. Staggered Line. They outnumber us three to one!"
Half-Jaw: "Then it is an even fight."
- Infernal Retaliation: When a Brute Chieftain's charging at you, roaring, and brandishing a gravity hammer, you might think of hitting them with a firebomb, as it does a lot of damage. Unfortunately, Chieftains have equipment that renders them temporarily invincible, so when they activate it, they're now charging at you, roaring, brandishing a gravity hammer, on fire, and invulnerable.
- In the Back: How the Prophet of Truth kills Miranda Keyes. And how the Arbiter kills Truth.
- It Has Been an Honor:
Cortana: If we don't make it...
Master Chief: We'll make it.
Cortana: It's been an honor serving with you, John.
- Which itself is a Call Back to the first Halo game, when returning to the Pillar of Autumn in a captured, damaged Banshee.
- It is also worth pointing out that this is the only time in the game series to this point that Cortana has used the Chief's given name.
- Large Ham: Even more rampant than in Halo 2, with Sgt. Johnson, the shipmaster, Gravemind and a hammy Prophet of Truth. The latter two even engage in Ham-to-Ham Combat during the first climax.
- Mythology Gag: Truth's speech in the second level ends with a quote from the first game's E3 2000 trailer, albeit altered to relate to Truth instead of the entire Covenant.
"Your destruction is the will of the gods! And I? I am their instrument!"
- Dual-wielding was disabled for the Needler, and its magazine size was decreased. The needles were also changed to do all of their damage right when they hit a player (rather than doing most of it when they shattered), travel faster, and require one less hit for a supercombine.
- Dual-wielding itself was also nerfed for most weapons, especially with the Magnum; dual-wielded Magnums take almost twice as many shots to kill as one on its own.
- Never Found the Body: Master Chief. Earth assumes he's dead.
- Not So Harmless: Guilty Spark.
- Outrun the Fireball: Blowing up Halo. Again.
- Red Eye Take Warning: 343 Guilty Spark, oh so much.
- Shut UP, Hannibal:
Prophet of Truth: I am Truth! The voice.... of the Covenant!
Arbiter: And so, you must be silenced.
- This Cannot Be!: Guilty Spark upon learning you intend to destroy the Ring.
343 Guilty Spark: UNACCEPTABLE! UNACCEPTABLE! ABSOLUTELY UNACCEPTABLE! Protocol dictates action! I see now that helping you was wrong! You are a child of my makers, inheritors of all they left behind. You are Forerunner, but this ring... is mine!
- The Unfought: Truth and Gravemind.
- Villain Decay: The Prophet of Truth goes from a Magnificent Bastard to.. Well, the Prophet of Regret.
- Villainous Breakdown: Truth, Gravemind and Guilty Spark ends up with this. Spark's crackdown is the scariest one.