Halo: Combat Evolved/343 Guilty Spark

You are dropped of at a dark swamp with an Assault Rifle, a Pistol, and full grenades. You should see another Pelican in the distance, just off to your left. Walk to it. If you're playing on easy mode you should find two shotguns and plenty of ammo for them nearby. You don't need to grab one, but it is highly recommended. On any level, there are three Assault Rifles (one is on top of the weird-mushroom things behind the Pelican). Picking extra AR ammo up is also highly recommended. Head out into the swamp, away from the back of the Pelican. Some Grunts and Jackals will appear. These two types of enemies will be the only one's you will be seeing for a while. Kill them. After that try looking around for a Covenant Dropship lying against some rocks. Go around or through it and you should find some more Covenant.

After you take them out, you should see an enormous root-like thing with an Assault Rifle at its base. It's a bridge, so go across it. If you fall off, just circle back to the Pelican and try again (there won't be any more Covies this time around, so it should be a little easier). Upon crossing, you should see a stationary gun to your right and strange shapes in the fog. Climb either to where the gun is or to the left. You will be greeted with the sight of some Grunts and Jackals running for their lives, a torrent assault rifle fire, and an explosion which flips the stationary gun below when you come closer. Ignore all of that (Covie's included) and continue on. Even if you are spotted by them, you can simply run inside the structure, which they won't follow you into. No surprise, as they just ran out of it.

Inside, you should see a lift. Go down. Straight in front of you is where you need to go. The wall you see is open on either the left or right sides, but there's a large amount of Covenant soldiers blocking it. Clear them out with a grenade, then finish the stragglers. Go through the door and you should find some more Covenant; still the usual Grunt and Jackal combination. Kill the ones on the platform you are on, then either snipe the ones below or jump down and finish them. The door near the Covenant is where you need to go. Note that the doors on this level with green lights will open for you. Going through the door, you run into a wall. Go either left or right. The next door you come to will put you directly in front of a crazy marine with a pistol, babbling stuff. He will shoot at you, but just stay out of his way. He won't follow you. If you want to hear what he's saying, you can just stand behind the pillar or wall thing he's backed up against.

You should find some weapons lying around here, usually Covenant plasma weapons. After you're done, head to the back of the room (opposite where the Marine is) and jump onto the platform above the fire. Run around to the door on the other end (the one with the Marine) and go through. This is another good place to hear the Marine's ranting. Through the door and the hallway beyond is a light bridge. You can either activate it or go around the platform sides to the door at the other end. There is an Assault Rifle on the floor. Go in the door. Go to either the right or left (doesn't matter, they both end up in the same place). The left path has an Assault Rifle and a med kit, if you need either. Walk down the ramps to the door below.

After the cut scene, go to your left if you need ammo or weapons, then go to the door opposite where you start from. Wait for the same monstrosities from the cutscene to break it down. These are Infection Forms, the first stage in the horror that is the Flood (as in the chapter title). They aren't very strong (a single round from any weapon will pop them, which will in turn pop any others within range), but there are a whole lot of them. Once they break down the door, throw a grenade into their midst (be careful with Plasma Grenades, as you might tag one and get it in your face). Kill any survivors with normal weapons fire. The rest of the rooms will now burst open. The sequence is as follows (from the first door):
 * 1) Left back
 * 2) Right forward
 * 3) Right back
 * 4) Left forward
 * 5) Directly behind (the door you came in through)