In the endless conflict raging across the galaxy, the weapons of conquest are turned on the Kaurava system. There the Dark Eldar enter the fray. Once part of the Eldar empire, they indulged themselves in all manner of forbidden and profane desires. Their desire to inflict pain and torment brought birth to the Chaos God Slaanesh which ravaged thousands of Eldar worlds and corrupted millions of Eldar. Once Slaanesh had won the war, he lay upon the Dark Eldar a curse. He condemned them to drink the souls of other beings in order to satisfy their thirst.
Attracted by the stench of the Dark Eldar, the second new playable faction enter the scene: the Sister of Battle whose primary concern is to purge the universe from heretics and rogue psykers, as they view them as unclean abominations.
Both factions bring new units, buildings and special abilities to the battlefield. On the side of the Sister of Battle this includes heat and flame-based weapons as well as spells like Holy Edict which stuns a specific enemy squad. The Dark Elder on the other hand make use of more gruesome armies which specialize in torture and soul sucking.
The add-on also adds a new unit to every one of the other seven factions while the core gameplay of the series remains intact and unchanged.