Typhus advances backed by his trusted Death Guard Chaos Space Marines.

Ten thousand years ago, the Imperium of Man was beset by a devastating civil war known as the Horus Heresy. Several Space Marine Legions rebelled against the "False Emperor" under the persuasive-yet-corrupt leadership of Warmaster Horus, a former lieutenant of the Emperor who fell under Chaos' sway. Space Marine fought brother Space Marine, culminating in a final battle at the Imperial Palace on Holy Terra.

Though the forces of the Imperium were victorious, the Emperor of Mankind was mortally wounded and encased in the Golden Throne to sustain his life. Meanwhile, the surviving rebel Space Marines fled for the Eye of Terror, a massive Warp storm on the northeastern edge of the galaxy. Here, warpspace mixed with realspace and the Daemons of Chaos could manifest themselves.

The traitor Space Marines took refuge in the Eye of Terror, launching raids and Black Crusades from their maelstrom base. These Chaos Space Marines became an endless plague upon the Imperium, wreaking havoc in the name of their Chaos Gods.

Chaos Space Marines have most of the capabilities of regular Space Marines, though they also have Chaos abilities and gear that make them unique. Each of the four Chaos Gods have Chaos Space Marines devoted to their cult, be they Noise Marines in service of Slaanesh to Plague Marines in service of Nurgle. They can unleash untold horrors on their opponents, from Daemon Packs to Chaos Defilers. They can be organized in an infinite number of ways, which is appropriate because Chaos embodies disorder itself. In the end, Chaos Space Marine armies are feared opponents that are seen as highly unpredictable by those who fight them.