Warhammer 40,000: Dawn of War/Chaos Space Marines

Chaos Lord
Close combat unit that can attach to squads to boost morale.
 * Class: Commander
 * Role: Melee, support

Chaos Sorcerer
Powerful damage and movement restricting special abilities. Can teleport short distances.
 * Class: Commander
 * Role: Support

Heretic
Has the ability to turn health into increased construction power.
 * Class: Infantry
 * Role: Builder unit

Cultist Squad
Weak unit with high numbers useful as a meat shield. Can be upgraded to detect infiltrated units, have plasma guns and Grenade launches.
 * Class: Infantry
 * Role: Anti-infantry, cannon fodder

Chaos Space Marines
Similar to standard Space Marines however tainted by Chaos. Can be upgraded to have heavy bolters and plasma guns.
 * Class: Heavy infantry
 * Role: Versatile

Raptor Squad
Close combat unit capable of short distance flying.
 * Class: Heavy infantry
 * Role: Jump infantry

Horror Squad
Ranged attack daemon summoned from the warp into the map.
 * Class: Infantry
 * Role: Anti-infantry

Possessed Squad
"The ultimate combat unit" with a variety of upgrades.
 * Class: Heavy infantry
 * Role: Melee

Obliterator Squad
A squad of pure destruction that can lay waste to forces by using a variety of weaponry.
 * Class: Heavy infantry
 * Role: Ranged firepower

Chaos Rhino Transport
This is just like the Space Marine transport, only with spikes. The enemy doesn't seem to use these much in the campaign.
 * Class: Vehicle
 * Role: Transport

Defiler
A daemon-possessed, four-legged walker vehicle. The Defiler has anti-infantry weaponry, a strong melee attack and an (inaccurate) battle cannon that will disrupt/demoralize infantry and deal decent damage to vehicles and buildings - a "Jack of all trades". It's best to keep your distance and take it out with lascannon vehicles - a missile squad will do, but may get knocked around a bit before succeeding.
 * Class: Vehicle
 * Role: Versatile

Chaos Predator
Equivalent to the Space Marine Predator. Starts out with anti-infantry weapons and can be upgraded to lascannons. Pretty durable so it needs some focused anti-vehicle fire (or Dreadnoughts if you can get them close enough).
 * Class: Vehicle
 * Role: Main battle tank

Bloodthirster
This is the Relic unit for Chaos. It's a greater daemon who's a dedicated (giant) melee fighter with jump ability. The Bloodthirster is built from the Daemon Pit, but doesn't actually appear there. Instead he's activated from a commander or squad leader unit who is then killed and replaced by the Bloodthirster. Hence one of these could show up unexpectedly in the middle of a fight. Killing one isn't much different from killing an Avatar; you dance with, say, an Assault Squad while focused fire wears it down. Your commanders can hurt it pretty good as well.
 * Class: Daemon
 * Role: Melee

Daemon Prince
The ultimate Chaos unit, a hero of Chaos "ascended" to a position second only to the Chaos gods. Treat him like a tougher version of the Bloodthirster. Of course, killing the Daemon Prince will finish the whole game so feel free to mob him with all your melee units and keep dropping Dreadnoughts on him as fast as you can build new ones.
 * Class: Daemon
 * Role: Melee