Black Legion:
This Legion is the ‘Vanilla’ Chaos Space Marines. It has the most options, and has access to every unit in the Chaos Space Marine Codex. A Black Legion army can be good for close combat, shooting, or a mix of the two. It’s the most versatile of all the Legions, and is good to start out with. If you are unsure what Legion to start with, then Black Legion is probably for you because it gives you the ability to try out most things in the codex, and lets you get a feel for the general Chaos army.
Alpha Legion:
Alpha Legion focuses on infiltrating. You get the Infiltrate special ability at a cheaper price for some units, and access to Chaos Cultists, a hoard type unit. As the Alpha Legion is an undivided force, it may only take the Mark of Chaos Undivided. Alpha Legion has access to Daemons, which is different compared to some of the other undivided legions of Chaos. Typically, Alpha Legion is a more infantry base list mainly because of their access to Chaos Cultists. They generally have a mix of shooting and close combat units. If you like a sneaky, infiltrating infantry army, then this Legion is for you.
World Bearers:
World Bearers are an infantry, and daemon heavy force with access to all types of daemons. They may get extra troop slots, but have to give up an Elite, Heavy Support, and Fast Attack choice. The World Bearers can be lead by Dark Apostles that makes the units around him fearless. Unfortunately, the World Bearers can’t take specialized marked squads. If you like Daemons, this is the Legion for you.
Iron Warriors:
The Iron Warriors probably have the most firepower compared to the other legions. They gain an extra Heavy Support Choice, and the limitation on Obliterators is removed! You also gain access to the Basilisk (see IG codex) and Vindicator (see SM codex) tanks. You also gain access to the Servo Arm, which acts similar to the Space Marine Tech arm. . However, they must sacrifice two Fast Attack options to gain the heavy support selection, as well as not being able to select Daemons or specifically marked units. If you like firepower and ordnance then Iron Warriors are for you.
Night Lords:
The Night Lords are swift and stealthy. By giving up 2 Heavy Support choice, they gain one fast attack choice. A Night Lord army can be fast and hard hitting with many Bikers and Raptors. They lose access to Daemons exept Furies and God specific marks however. They can also take the stealth adept veteran skill. If you want a quick, and mobile force that can hit fast and hard, then Night Lords may be for you.
Khorne (World Eaters):
Khorne is a close combat oriented legion that serves the blood god. Their main daemons are the Bloodletters, and the Bloodthirster. Their main troop choice is Berserkers. Basically, they charge forward and try to get into assault, usually backed up by a Predator for some fire support. They have limited firepower, but they make up for it in close combat. The Sacred number is 8. If you like crazy blood frenzy units charging for close combat, then World Eaters may be for you.
Nurgle (Death Guard):
The Mark of Nurgle gives an extra toughness, gets the True Grit ability, and makes them Fearless. Many forces have infiltrating/rhino squads with melta/plasma guns blast away, and then assault usually accompanied with an AC. The main daemon of nurgle is Plague Bearers, and their main troops are Plague Marines. They are very resilient, and are overall a powerful army with 7 as their sacred number. If you like resilient, a powerful units, then Death Guard may be for you.
Slaanesh (Emperors Children):
Emperors Children serve the god of Slaanesh. They utilize sonic weaponry that can be very powerful. The Noise Marines are excellent at shooting and assault. Daemonettes are the daemons of slaanesh, and though they can be fragile they can be very devastating on the charge. Slaanesh are considered by some to be the best legion for doing the ‘Daemon Bomb’ tactic. There sacred number is 6.Accompanied by their good psychic, shooting, and assault abilities the Emperors Children can be a powerful, and fun legion to play.
Tzeentch (Thousand Sons):
The Tzeentch have greater sorcerers, and powerful spells. They are granted an extra wound, but move slow. Because of this, Thousand Son armies tend to be slow, shooting and psychic oriented armies. Their Daemons can be impressive, by laying down lots of firepower.
Their sacred number is 9.If you like resilent, and psychic powers coupled with a lot of firepower, then the Thousand Sons might be for you.