3rd Edition Rulebook + Non-Chaos Codices Chaos Lore (1998-2004)

If I were to attempt to take all the chaos excerpts from this book it would effectively be the whole thing, I recommend finding a copy and giving it a look through.
 
A Slaaneshi-worshipping cult has engaged in all manner of debaucheries to send a psychic signal into the Warp to attract followers of their daemonic patron, Instead, the Tyranids have arrived and the cull members must be killed to sever the signal and prevent more from arriving.


THE GREY KNIGHTS
On Saturn's moon of Titan, a massive basalt fortress- monastery tears from the ice above the oceans of liquid methane like a jagged, black spire. The monastery is home to the most secret of Chapters in the Imperium, the Chamber Militant of the Ordo Malleus, the Grey Knights. The dusty, echoing halls of this fortress are hung with battle honours stretching back almost ten thousand years, though none outside the Grey Knights or Ordo Malleus would recognise the names of the conflicts inscribed in the faded gold lettering.

Legend tells that it was around the time of the Second Founding that the Emperor ordered the creation of this secret Chapter of Space Marines. The fragile Imperium had only just survived the galactic civil war of the Horus Heresy, and was still very much at the mercy of the powers of Chaos. The Emperor understood that it would require a dedicated band of incorruptible warriors to protect it from the dread creatures of Chaos in the days to come, and so the creation of the Grey Knights was undertaken in great secrecy. Where other Space Marine Chapters were created from the gene-seed of existing Chapters, the Grey Knights were unique in that their gene-seed was said by some to have come from the Emperor's own flesh. The Ordo Malleus was in its infancy at this time, the corruption of Horus lending new impetus to the creation of an order tasked with the hunting and elimination of the daemonic.

The Grey Knights were permanently attached to the Ordo Malleus, becoming the hammer with which these servants of the Emperor would smite the forces of Chaos, and one of its Grand Masters is traditionally a member of the Inner Conclave of the Inquisition. Space Marine recruits are often drawn from feral words and the Grey Knights are no exception; its recruits are emergent psykers plucked from the most savage warrior cultures imaginable, where only the bravest and strongest survive. Recruits selected to join the Chapter are returned to Titan to begin the long and arduous process which weeds out those without the physical strength and immense mental fortitude to become a Grey Knight. Only the best of the potential recruits survive this selection procedure and are deemed worthy to begin the process of transformation from human to Space Marine. Once their bodies are implanted with the precious gene-seed, the aspirants’ true training can begin.
 
He felt a surge of justifiable pride in his ability to defeat the spawn of chaos He swiftly suppressed it. Pride was one of the six hundred and sixty six sins which enabled the spawn of the Warp to control their mortal pawns, Had not the Warmaster himself, greatest of the Emperor's Chosen Primarchs, fallen victim to pride? It had been Horus's folly to believe that he could master Chaos rather than be its slave, Stern knew this to be slicer madness, The Daemons of chaos acknowledged no masters save the four Great Powers themselves, and what were those except Daemons grows mighty beyond mortal reckoning?

The confrontations that followed were fought bitterly, first one side then the other gaining an advantage, until finally Quixos and Helgrund clashed in personal combat. Heljrund wielded a mighty force hammer, charged with faith and purity, while Quixes's daemonblade contained rite ferocity of Kharnagar the Deathly, a being he had defeated some decades earlier and whose essence had been bound into a sword during is recent experiments. The two battled long and hard, some Inquisitors having since likened it to the fight between Horus and the Emperor at the birth of the Imperium.