Watch Your Pf's and Jj'sThe world that Bungie created for Marathon is very detailed. The back story for the games is conveyed through the use of terminals on the ship. Through entries in crewmen and system logs, you will watch the story unfold as you fight your way through the hordes of aliens to be found. Here's a brief guide to what you will find and how you will dispatch them.
Pfhor: This is the most common of the aliens that you will face. Standing upright, the pfhor are tall, thin aliens with three eyes.
The Fighters are lightly armored, and carry staff weapons. They come in four varieties, denoted by colors. The green Fighters are the weakest, the purple are stronger, the orange even stronger, and the blue are the strongest Fighters. The blue Fighters are also capable of firing projectiles from their staff weapons.
Troopers are more heavily armored, and carry a much more deadly auto-rifle / grenade launcher combo weapon. Like the Fighters, the Troopers come in two varieties denoted by color: green and purple.
Hunters are very heavily armored with shoulder-mounted energy weapons. They come in three colors: brown, green, and blue.
Enforcers are strange, cloaked aliens possessing alien weapons. This type of Pfhor comes in two varieties: green/blue and blue/orange.
The biggest and baddest of the Pfhor forces are the Juggernauts (also known as "The Big Floaty Thing What Kicks Our A**es"). These are huge gunships armed with rocket propelled grenade launchers and machine guns. While these foes are formidable in any form, they also come in two varieties: grey and brown.
S'pht: The Pfhor use enslaved races to enbolden their ranks. One of these races is the cybernetically enhanced S'pht. These floating cyborgs look like creepy monks. They are armed with a built-in pulse weapon, and some are actually invisible. Before they were enslaved, the S'pht believed that sentience was impossible without cybernetic implants.
Hulks: Also known as the "Drinniol," this is another race enslaved by the Phfor and forced to fight for it. Although they are only featured in a single level of the first Marathon game, they are notable due to their huge size and massive amount of damage that they can inflict. Although their slow speed makes them easy to avoid, they take a large number of hits to take down, and one lucky hit from them can take out most of your energy.
F'lickta - Marathon: Durandal saw the introduction of the F'lickta, a race indigenous to the planet Lh'owon. These creatures are relatives of the S'pht, and often harass the Pfhor. They live in the water, the sewers, and in lava. While the water variety can only attack with their claws, the sewer and lava dwellers can lob sewage and lava respectively. Entering their domains is not recommended.
Jjaro - The Jjaro are a highly advanced race of creatures that disappeared from the galaxy millions of years ago. Although the Jjaro are never seen in Marathon, they are responsible for many aspects of the game, such as the technology of the Pfhor and the cybernetic implants of the S'pht. The Jjaro possessed a star-destroying weapon known as the Trih'Xeem, and had the ability to move entire planets by warping space around them.