Beat the game on the Legendary setting to unlock a longer, different final movie.
Alternate ending sequence
Successfully complete the last level of the game in campaign mode under the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both.
Defeating Flugs
In the later levels of the game you will encounter new enemies called Flugs. These enemies will be small at first, but they will attack and control anything alive. Use a shotgun on these baby Flug as they tend to group in large numbers. By using the shotgun it will send a storm of bullets and kill most of them that are around. Use a shotgun on the possessed Flug, and it will kill most of them in one shot. This is made easier by the fact that most of them carry shotguns.
Defeating Grunts
When the Grunts are sleeping you can beat them to death with the butt of your gun without waking up any of the other Grunts. You can usually kill a four to seven of them before an alien spots you. They start sleeping in level 5, Assault on the Control Room. Just remember not to shoot or they will wake up.
Defeating Jackels
Use a photon weapon to destroy a Jackel's shield quickly. When you encounter a Jackel, run up and hit it with your weapon or get out a gun that uses bullets, not energy.
Covenant dropship
When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area.
Assault On The Control Room: Spinning ring
At the Halo start up screen, choose multiplayer mode. Choose cooperative play at the next screen, then choose your profiles at the next. Choose the level where in the beginning it says, "I would have been your daddy" (Assault On The Control Room). On the difficulty screen, choose legendary. When the level starts after the introduction sequence, have one of the players quickly press X when the "Press X to get on the Pelican" message appears. That player will re-board the ship and start flying with it down the hole. The other player will quickly die because of the aliens, and will be respawned on the spinning ring. Make sure to jump where there are gaps. If you are good enough, you can even jump to a nearby ledge. Additionally, once you are respawned on the ring, quickly jump off and fall to your death. Sometimes you will be respawned on the lowest ledge in all the blue haze, and a "Press X to flip the Pelican" will appear that the top of the screen very briefly.
Unlock New Guns
To unlock two new guns, first beat the game on the easiest setting (Easy), and then on the hardest setting (Legendary) in under 3 hours.
Fireworks
Through one plasma grenade in a wide open spot, then stick another directly on top of it if you do this fast enough the first gernade will launch the second and vuaola fireworks.
Your in My Seat
When one of your computer controlled buddies is in a vehicle spot/seat you want simply go over to that spot and press X. They will graciously move.
Infinite Grenades
To get infinite grenades, scorpion, and ghost in multiplayer- Go to the multiplayer screen: go to edit game types: In that menu you will have the option to make up your own game type.
Shortcut
Play "Assault on the Control Room". It doesn't matter what difficulty setting or if you are playing 1 or 2 player mode. Get to "If I only had a super weapon". Rush right ahead and ignore the Jackals and the first Elite. Instead, kill the second one as he is somind over the hill. Then run to the next one who is coming around the banshee. If you kill them both, the two banshees will be yours for the taking. I would suggest using the Plasma Rifle for killing them, as it stuns. You can use the 2 banshees for a banshee fight with a friend, too. Anyways. Ignore the enemies and do not go to the end of the bridge. Instead, go to the top of the hill that leads to the Control Room. There will be no Hunters, or aliens on the way up the hill! There will be the enemies that come out of the big door, though, but this shave's precious time off the level.
How to get a Banshee sooner than the bridge
Assault on the control room: How to get a Banshee sooner than the bridge, 1 player.
Fight your way through the level to the first snowy open area, kill the enemy tank and all the rest of your enemies then hop into the Warthog. Make sure you get the Rocket Launcher and the Sniper Rifle. The next open area has a battle raging and an empty Scorpion, hop into the Scorpion and waste all the Covenant with the cannon. To the left is tunnel, to the right is the obvious way to the next open area, go to the right and clear the enemies around the corner. Stop (If you kill all the enemies in the next open area an Elite will get the Banshee before you do and use it to attack you. You'll know you're in the right place if there's another enemy tank in the area.) Now, jump out of the Scorpion and stay to the Left edge of the valley. Look up, you should see a Ledge above you with some lights on the underside. The Banshee is on that ledge so get underneath it and shoot some rockets up to knock it off. Remember to move something forward with concussion weapons you have to shoot behind it, so use you brain. If done correctly off will tumble the Banshee, if done incorrectly it will be pushed farther in. As an added bonus the last few sections of the level will have no enemies due to the fact you've skipped so much.
Plasma Grenade
If your going to use a plasma grenade, try throwing the grenade on the alien where it sticks onto them. It will blow up on them making sure it will kill the alien. Also the aliens can do this to you if you don't move.
Name in Blood
In the very last level of the game, as you're driving the Warthog out of the Pillar of Autumn, on the second jump, do not go all the way across. Then back into the room thats behind you and get out. if you look on the wall the name REX will be in blood on the wall.
Hungry Grunt
In the very last level of the game, as you're driving the Warthog out of the Pillar of Autumn, get out at the 2 tunnel off of the main one and there will be a grunt at the end of it. he should say something like " I can't wait to get back to the ship and eat some grub!"
Good tactic for legendary
Here's a good way of killing elites or hunters in legendary. get a needler and some ammo and just unload it on an elite and watch the show --NOTE-- You have to get in close enough for the needles to find their mark, and the delay until they explode leaves you wide open for their plasma rifle, so bring a backup weapon.
For the hunters, unload an entire clip at their backs so the shot doesn't reflect off their shield and start shooting at them with another weapon while waiting for the needles to explode (or just sneak up on one and smack him in the back, and if you cant to that...a well placed plasma grenade to the head should weaken it, but it will most likely just piss it off.)
Silent Level shortcut
This will help if your annoyed by always having to go back because the yellow dude locks the door. First find a warthhog and get some people inside. Go where the place where there are lots of guys and where kortana says the covenant are putting up a real fight. Go down completely with your warthhog and go at full speed directly where the yellow dude is. If you did it right then your warthhog will be half way through. Press forward and get out. Then pass the level like you usually do and come back where the warthog is stuck. You may think it is impossible to get out but just press back and then get out. You will be in back of it and now you're done the hard part!
Elite Kill Trick
In the levels where you must face the Covenant there are certain aliens called Elites. They are the big ones which are usually blue, red, or sometimes yellow. If you have a gun with a scope ie; pistol or sniper rifle,try to get behind an elite without being seen by it. Next, Zoom in at the back of it, you will see yellow spots on his back or his head. If you shoot them a yellow liquid sprays out and they die instantly. This cheat helps because you only need to use 1 bullet and you do not have to get in close to kill it. It also looks cool when the yellow stuff sprays out.
Kill Master Chief in Cinematic
This is only possible to do in co-op mode, so get a friend to help out.
Near the end of "Assault on the Control Room", at the base of the big pyramid-type building, rush over and steal a Ghost from the two parked ones.
Get the Ghost (in one piece) to the top of the pyramid (Get your friend to do the fighting while you take care of the Ghost). Open the large blast doors, kill whatever is in your way, and keep going down the hallway.
When you reach the VERY LAST door (ie. The one that triggers the cinematic), stop. Line the Ghost up with the centre of the door as best as you can, and back it up about one or two Ghost-lengths away. Have your friend hit the switch to open the door, wait a half-second or so, and then gun it towards the door.
If your timing was right, the Ghost should JUST fit between the doors, and during the cinematic the Ghost (now without a pilot) will mow down Master Chief from behind, killing him. The cinematic will go on as normal, except for the absence of Master Chief (you will still hear his voice, though). Also, in certain camera angles, you will see Master Chief's corpse in different poses, such as tilting 45 degrees to the left with his gun stuck in the ground.
It will work more than once (I've done it three times so far), and it just looks damn funny to see Master Chief get run down by his own Ghost. Props to The Fiend for co-piloting.
The Yeti
This isn't so much a code. It's just something that shouldn't be where it is. In the level "Guilty Spark 343", there is a crashed Covenant dropship just around one of the corners you must turn (the second or third, I believe). Up above this dropship is a tree jutting out over a ledge. On this object is a Flood-infested Elite (the bulkier Flood ones that don't pop). It appears as you come around the corner and runs back along the log after a few seconds then disappears. It is named 'The Yeti' because it diappears whenever seen. This is really weird because the Flood have yet to be introduced at this point. Also, the Yeti is invincible. You can sneak up on it from behind (which is too long and tedious a subject to go into), but don't have your co-op partner go around the partner until you're in position. It can withstand a pistol whip to the back of the head and a plasma grednade, so we just gave up. It's really fun, though. I suggest you try it.
