Sierra 117 is the second level of Halo 3's campaign. It is the first level to feature player-influenced gameplay.
 Enemy Ground Force
 Enemy Vehicular Support
 Available Weaponry and Equipment
- Plasma Pistol
- Plasma Rifle
- Beam Rifle (Heroic and Legendary only)
- Brute Shot
- Gravity Hammer
- Plasma Grenade
- Assault Rifle
- Battle Rifle
- Sniper Rifle
- Fragmentation Grenade
- Bubble Shield
The first level in Halo 3, Sierra 117 starts with a narrative from our good friend, Cortana. There is a ship crash landing and the Chief has managed to jettison himself. Just as Cortana is talking about how lucky Master Chief is and how she picked him herself. A bittersweet moment, to say the least, as Cortana is now in possession of the Gravemind from the end of Halo 2. After you see him crash landing in the jungle, you'll see his stiff body lying on the floor. His suit has tensed to protect him inside. After a tense moment of not knowing whether or not the Chief will wake up, he recovers from the trauma and gets back to work. They need evacuation from the swamp, first and foremost.
 Walk it Off
The level starts with the Master Chief waking up in the middle of the swamp of Sierra 117 with Sergant Johnson and The Arbiter. Once you're back on your feet, you have to make your way to an evacuation location. Follow the path out of the the clearing you started in and around the corner, up a few rocks. You will come into another clearing with a waterfall. Climb up on the rocks there and you will hear a Phantom coming in, as well as see some Brutes in the distance. You have the element of surprise here, so take it easy. When the small pond in front of you is full of Grunts, open fire with your Magnum. Aim for the head for quick kills, then you and your support team can handle the Brute. Follow through the clearing until 2 Phantoms hover in front of you. Take cover from them until they drop their reinforcements and leave. The guns will rip you up. Move forward and use your elevated position as an advantage over the Brutes and Grunts below. When you drop down, more Grunts and Brutes will come out from the back of the clearing. Take them out. Using your Pistol with a plasma pistol secondary is an easy way of doing it, because a charged burst of the plasma pistol can take out a brute's shield, then finish him with a headshot from your pistol.
Continue out the back of the clearing and drop down to the next elevated place with ridges all around. If you're playing on Legendary, be very careful this time. Jackals can be deadly on Legendary. They will kill you in one shot most times, so tread carefully. Take them out one by one from cover as they shoot at your team mates. There is only a couple this time, so take aim mostly for the Grunts below. Use the Fusion cores to take out groups of them, then when you've cleared enough space, drop down into the middle and take out any stragglers. More Jackals with Carbines and Brutes will spawn at the back of the clearing, so deal with those before moving on.
When you move on from this clearing, Johnson will tell you you need to bring Bravo team to the evacuation point if you find them. As you round the next corner, look high, you will see a Brute holding a UNSC soldier by the throat. Stick him to save the soldier and take out any surrounding forces. Continue up the back wall and across the log where you saw the Brute holding the soldier. More Jackals will spawn across the log, as well as more Grunts on the left and even more Jackals up the hill. On the left you can find 2 dead Human soldiers. If you desire, you can take a Magnum, Battle Rifle or an Assault Rifle from them, then continue on up the hill and down into the ensuing cave. Be careful of the Brute that hops out. He will drop a Bubble Shield.
Half way through the cave, you will have a vision of Cortana saying, "Would you sacrifice me to complete your mission? Would you watch me die?". It seems she is being telepathically beamed into the chief's head, but how..?
 Charlie Foxtrot
Continue out of the cave with the Arbiter. You're at the evacuation site now, but this is Halo. It's not that easy. The Pelicans will be in a battle against ground forces of Covenant and Brutes. After popping a few Grunts, you will hear one of the Pelicans sending out a distress signal, as they are being attacked by Banshees. They will both be blown up, so you have to find a new extraction point. First of all, you have to fight through the Covenant forces on the ground. Use headshots to quickly dispatch any grunts on the ground, and use a Plasma Pistol/Battle Rifle or Carbine combination to take out any Brutes in the area. After a Phantom drops more reinforcements and you handle them, you will see large concrete pipes at the back of the clearing. Head up there and you'll encounter more Jackal/Grunt/Brute resistance. Be careful of the Brutes, because they now have Brute shots. Take them out and continue out the back. Be careful of the last group of Jackals that attack you as you leave.
Now you're in the "Jackal Forest". This is the section I mentioned earlier where all the Jackals are equipped with Beam Rifles and can easily take you out. If you take your time, watch the ledges carefully and listen for any shots fired, you will easily come through the forest unscathed. Stay as high as you can.
Bonus Easter Egg: If you keep to the left at the dog-leg bend at the end of the Jackal Forest, you can jump up to where one of the jackals is standing after you kill him, and you can find a small family of cave men. They are not alive nor are they dead. They are simply dummies, put in as an Easter Egg.
The next checkpoint will lead you out at the top of a large crevice. There will be a lot of Brutes and Grunts down in the valley, so keep the high ground until you have taken out any of the remaining Jackals on the ridges around the clearing. Then drop down and use the thin valley as a bottle neck and as cover against the Brutes and Grunts. When you get past the initial wave and you round the corner, you will encounter more Jackals in trees and another Phantom. If you drop down where the Phantom flies in, you can find a Battle Rifle and a Human Sniper Rifle. Take them and continue up the hill. More Jackals and Grunts will come around the corner throwing grenades everywhere. Take them out and continue around the corner and up the hill. Another small cave will lead you to a large compound and the final part of the level.
 Quid Pro Quo
You are now at the final extraction point of the level. Although it won't be easy. The installation is heavily fortified with Jackals, Grunts and Brutes, both regular and Zealot Brutes, wielding Gravity Hammers. Clear the immediate area underneath you. The high ground is not where you want to be in this instance as you are in a bottleneck, so the vast number of enemies below are now focusing their fire all on you. Drop onto the roof where the jackal is and through the break in the roof. You are now somewhat safe. There are no enemies behind you and the enemies across the other side of the installation cannot see you inside the building. You only need to worry about the enemies at the back of the building ahead of you. Take them out as they come out the door. You will need your Sniper Rifle in this bit. When you round the corner, you will see a lot of Brutes at the end of the the long walkway. For now, ignore the Chieftan until he gets closer to you. He is a threat, but not at long range. His Gravity Hammer is useless at long range, so take out the ones with Carbines and Brute Shots first, then deal with the big Chief. Use a Bubble Shield for cover if you need to. If you let the Chief get too close to you before you take him out and you don't kill him quick enough, he will go invincible. Try to snipe him quickly, because you will have a tougher time with trying to back-track until he's vulnerable again if you don't. Continue across the walkway after you've finished with them.
One last piece to the puzzle now. Kill any stragglers and, when across the walkway, climb up on the pipes and head towards the building with the glass roof. Jump through the glass and open up the holding cells inside. This is Bravo Team, and now that you've saved them you need to head back to the walkway, where a Pelican will arrive to extract you and your team. Once you take out a final group of Grunts and Brutes by using the ramp at your end of the walkway as cover, you can climb up onto the box in the middle of the bridge and jump up into the Pelican for extraction, which will end the level.
Now, it's on to the second level, Crow's Nest.
 Hidden Skulls
 Blind (Silver)
The Blind Skull is sitting on a stone outcropping near the beginning of the level. After eliminating all of the covenant forces in the first waterfall area, drop down to the basin (where the Phantom drops its reinforcements) and take a right. Cut through the underbrush and the outcropping should appear on the immediate left.
 Iron (Gold)
The Iron Skull is sitting in a small rooftop alcove near the end of the level. After crossing the dam, jump onto the machinery located on the right side of the opposite landing. Continue along the ledge running along the front of the generator building until it terminates at a small alcove to the immediate right.
Walkthrough in progress!