Sacred Icon Walkthrough
From Halopedia, the Halo wiki
The player starts out with a Covenant Carbine and a Plasma Pistol. To the left is a pair of Needlers which are recommended because they are very effective against the Sentinels. A swarm of harmless Constructors is seen flying about zapping their repair beams before flying further ahead. Head towards the lighted section of this room and on the left will be a piston. A cowardly Grunt Major will be hiding in a corner, occasionally he will follow you. If he does give him a needler. Activate the piston and then jump down. (On higher difficulties the pistons are activated by shooting or melee). Beneath it is another Piston and Sentinel Launchers. At the end of the hall will be another piston. Active it and jump down.
In the next room are two Jackal Majors and a Grunt to aide. A glitch allows the Jackals to be given Covenant Carbines. Destroy all the Sentinels and Sentinel Launchers in the room. At the far end of the room will be a walkway that has triangle supports. Tartarus says that many of his Brutes have died fighting the Sentinels and the corpse of one is visible on the ground as the first Brute encountered in-game. Go down that hallway and you'll come to a room identical to the one you just came from. Once you've killed all the sentinels, there will be a piston at the far end of the room on the right side. Drop down there; the Jackals and Grunt will jump down to follow you. On entering the new room, a new scared Grunt will join up with your group, begging you not to send it back to fight the "big scary thing". Continue down the hallway, destroying sentinels and blowing up the things in the wall they come out of. If you're really concerned, make sure to run ahead and take out the sentinels and their launchers ahead of your troops, so your squad doesn't end up taking a lot of damage. At the end of the hallway will be yet another piston, so jump down.
You'll drop down into the shield door that's guarded by an Enforcer Sentinel. The Enforcer will usually end up killing most or all of your troops (2 Jackal Majors and 2 Grunts at this point, if you've been careful) with its barrage of artillery fire, but if you're quick enough and lucky enough sometimes your whole squad survives. Once you kill him (it will die when you unlock the plug locks first, and that will make it a lot easier on Legendary) go up to any one of the large pillars and activate it and then do the same with the other 3. Once you do that a holographic panel will come up at the center of the thing you’re standing on. Go up to it and press "X." Take a breather and enjoy the music as the Gondola takes you to a room identical to the one you just came from.
After a few moments of peaceful riding, you can see new Gold Sentinels (Sentinel Majors) and Flood battling ahead. At this point your troops, if they're still alive, will follow you on the gondola into the next fight. If you had time to optimize guns, take the Needlers away from the Grunts and give them Plasma Rifles, as Needlers aren't very effective against charging Flood. The Phantom will only stay for a little while, so you need to act quickly. The Phantom will directly engage another Enforcer hovering in the middle of a large hangar which your gondola is going to park in: lots of normal Sentinels come out of the sides as well, lead by Major Sentinels, but there are Flood swarming behind them. It's a three-way fight, and the Sentinels might fire at you a bit but will mostly shoot at the numerous Flood, who will quickly overwhelm the outnumbered Sentinels). If you time it just right and reload many times (it's right after a checkpoint) you can conceivably throw a plasma grenade at the Enforcer while it's still fighting the Phantom, before the Flood appear, but this is really difficult, and mostly done so that the Enforcer's artillery barrage doesn't slaughter your troops. You could just wait for its back to be turned and unleash a duel-needler barrage on it. Kill as many Flood and Sentinels as you can.
If you take out the Enforcer quickly, the situation will quickly devolve into a chaotic all-out brawl between your troops, the dwindling Sentinels, and the oncoming horde of Flood. This is a protracted Flood-battle which might take a good two minutes. This is very difficult in the heat of battle, but somehow try to rip SMGs off of dead Flood, and give them to your Jackals, *ASAP*, preferably mere seconds into the fight. Your 2 Grunts are basically a write-off at this point: even assuming all of your troops survive the Enforcer's barrage and not even considering the Sentinels, the Flood will easily overwhelm any non-shielded enemies. It is thus actually possible for one or both of your Jackals to survive the entire battle if you arm them with SMGs, and they actually take some real heat off of you. Once in a blue moon, in the small time window you have for arming your Jackals with human weapons in the first few seconds of the fight, a Flood will drop a shotgun, and Jackals can be armed with shotguns, which are of course great against the Flood (documented evidence of a shotgun appearing in the same game that a Jackal is still alive, and the player successfully giving the Jackal the shotgun, is not available at this time). However, even if your SMG-armed Jackals survive, their AI won't let them follow you into the rest of the Sentinel Wall and they'll just hang around the gondola while you proceed inside (of course, you could just race ahead after engaging active camouflage and never even try to keep your troops alive, but the challenge of trying to "save" them is half the fun).
After a while, a large door will open and two Sentinels will come out. Go through the door and follow the hallway until the end where there will be a piston. Jump down. Continue down the hallway and turn right. Activate the piston and jump down. Right near the dead Human bodies there will be a Shotgun and some SMGs. I usually take the Shotgun. Continue down the Flood-infested hallway and at the end will be another piston, so press "X" and jump down.
In this massive room there will be plenty of ammo spread around, you just have to look a little. I don't even bother fighting, but you can do whatever. Turn left from the room that you dropped down from the piston and you'll find a near-identical room with some flood in it. At the end of the smaller room will be an opening into an identical, massive room. Again, there will be some ammo strewn about, so if you’re running low, quick grab some. Otherwise, jump down into the massive room and head left. You'll see a piston, so press "X" when your near to it and jump down.
The next area is very dark, and perfect for the Arbiter's camouflage. Go straight until you reach the stack of boxes, and then turn right. Keep going until you reach the end and walk on the lowered platform in the middle of the room. It's an identical hallway that you've been down before-the one with the triangle supports. It doesn't matter which way you take, but head to the end of the room like before, and go down the identical walkway. Once you're in the next room, head left and at the end of the room you'll see a piston in an alcove to the left. Activate it and jump down, and do the same with the next piston. You'll drop down into a room like the one you were in when you started the level, except there's a piston in the middle of it. Go up to the piston, activate it, and drop down. You'll drop down into an identical room, except this one has a battle raging on. I always pick up the sword near the piston in the middle of the room before I activate the piston and drop down, but you don't have to. You'll keep falling for what seems like forever. When you stop though, there's a large door a few meters away. Walk through the door, and past the bridge. Keep going until you reach a Covenant Sniper Tower. Fight the waves of Flood with the other Elites until you've killed all of the Flood. Behind the Shielded Plasma Cannon is a small tunnel, head through that tunnel. A battle will already be going on, so help them. Keep fighting until the cut scene starts. Congratulations, you've completed Sacred Icon!
Speed Run Normal Walkthrough
Alright, so you've got a Carbine and a Plasma Pistol. There is a container to your left with two Plasma Rifles. Dual-wield them in place of the Plasma Pistol and keep going through the piston on the next level to your left. (Get used to the piston idea). At this part, it doesn't matter. If you want speed, you can overload it with the Plasma Rifles, or save the battery and activate it manually. The rest of the time you have to use the Plasma Rifles.
Go down, and through the next piston. Once down, you will go through the hall. Use the Plasma Rifles to activate the next piston, as Sentinels will come out. Same thing with the next hall. Now, you come to a hall with Grunts and Jackals fighting off Sentinels. Only kill the Sentinels if you need to! You'll need the ammo. Now, there's another hallway. do the same thing. If you're running low on Plasma Rifle battery, there is a dead brute with a full Plasma Rifle up ahead.
Once at the end, turn left and overload the piston. Now, you'll come to an enforcer guarding the plug locks you have to overload. Ignore the Enforcer! It will just die once you finish the task. Once you've overloaded all the locks it's just a push-a-button task. Go invisible and push the button in the middle. The Enforcer will just explode once you do this. Thank god that's over. Now Tartarus will just pick me up in the next landing, right. Another Enforcer will come. Just kill the Enforcer really fast so that you can go on without having to fight the Flood and Sentinels. Now, if you threw away your dual-wielded Plasma Rifle to use the Carbine, pick it up if you can find it. Otherwise, just keep going through the tunnel that opens once you get to the top. Don't wait around in here, just run through and get in the piston.
If you wait, hordes of Flood will come out and kill you fairly fast. Now, once you get to the bottom, the Flood are your biggest problem. Just before you jump over the second half-wall, activate your camouflage . TIMING IS IMPORTANT! I've been killed many times when my camouflage wore off in the wrong place at the wrong time.Or,when you here the doors opening,then you may also active your camouflage. Now, another Flood infested hallway. And what do you know, we've also got marines to play with!
On your way out of the piston room, swap your Carbine for the Shotgun. If you have the Sputnik Skull, you can grenade jump out the window to your right and get to the marines first hand, this is a lot quicker. Through this hall, you want to throw grenades, preferably Fragmentation Grenade, behind you, to kill any chasing flood. While running forward through the tunnel, after about 5 seconds of running, activate your camouflage. There is a crap load of Flood in the next room. Once your camouflage wears off, you should be able to fight your way through the smaller amount of Flood Carrier Forms and limited Combat Forms. If your camouflage recharges while doing this, activate it o skip the carriers. go down the piston, and you're in another hall, and you'll see boxes ahead of you.
Throw a grenade ahead of you to take out some Flood, and fight your way through the rest. The piston will bring you down to a floor with a window to the outside where there is an Enforcer helping other Sentinels fight Flood. Ignore them and go into the other elevated hall. Jump in and activate your active camouflage. Run through the Flood and go to a similar hall to the right of you when you jump down, and use the Plasma Rifle to overload the piston. Now you drop to a room where there is no flood and just a piston in the middle of the room. Go down it to another room just like it with another piston in the same place, just with Flood and Sentinels. Throw a grenade to the Flood on your right. There is an Energy Sword to the right of the piston. Swap your Plasma Rifle for it after you overload the piston with it. Drop down, and you're finally in open space. Run through the first area with Flood and Elites and go through the tunnel. Now just finish off all of the Flood in this area with your shotgun and energy sword (this is what it was for) and you're halfway to the great prize, the Sacred Icon!