The Codicium Aeternum is a holy document within the Ordo Malleus which outlines the details regarding, and means of banishing, known demonic entities encountered by the Inquisition. This particular short outlines the known details regarding one such demon entity within Warhammer 40k lore. (A demon that I also just made up)
The Greater Demon of Khorne known as Santa Claus is a somewhat difficult subject to cover since his legacy predates our understandings of the Warp by a pretty fair margin. According to our earliest, most highly sealed records from the Dark Age of Technology, the demon known as Santa Claus is actually a Demon Lord of a fallen proto-faith Saint known as Saint Christof. Our somewhat incomplete records are dodgy at best regarding Saint Christof the man excluding that he sacrificed his life for our mighty Terran Lord to redeem all mortal souls from the vile clutches of Tzeench. Once a year, prior to descending into demon-hood that is, Saint Christof would return to life on his birthday from the egg of a rabbit and travel the Earth decapitating the heads of criminals and collecting them in his giant sack. Honestly, we find these records to be mostly gibberish, but there is some consistency to his tales as to determine a pattern for the demon's behavior. Most notably, Saint Christof possessed a noticeably impressive array of pysker powers which ranged from being able to convert water into wine, to telekinesis which used to levitate a sleigh into flight upon his travels. The death of Saint Christof is somewhat difficult to determine, but according to Terran legend he must have died sometime during the Dark Age of Technology given how he effectively disappears from our records.
With that having been written I think it best to elaborate upon what we can confirm for certainty regarding the demon known as Santa Clause. This particular creature is a Greater Demon servant of Khorne and prefers to wear an elaborate red suit with white fur upon his sleeves and collar as means of displaying his warp affiliation. Santa Claus is somewhat overweight and survivors from his encounters describe him as 'infectiously' cheerful.
Legends regarding the flying reindeer and sleigh vary between encounters yet his incredible pysker powers remain a constant no matter the person weaving the tale. And, regarding the varying and numerous sources for his tale… there are many. For example, the people of Fenris know him as, "The Sleigh Master," a wondering traveler who roves their world aiding the lost and returning those free of crimes and guilt back to their homes without harm. Those he finds guilty of serious crimes are typically punished by severity of offense. There are even legends of beheadings. But, I digress. Some of the more fascinating accounts of Santa Claus actually come from the Dark Eldar to whom he is known simply as, 'Arith Mor," The Red Death.
These stories are fascinating… and a bit ghoulish. Apparently, once a year… Santa Claus breaches the inner sanctity of Commorragh and for several uninterrupted hours goes on an unstoppable rampage killing as many Archons as he can possibly get his hands on. These Eldar are butchered in extremely cartoonish fashions leaving no doubts as to their mutilator. Some of the stories given to the Inquisition by some trustworthy Harlequins include disembowelment with entrails spread out across the room and covered in tacky ivy and sparkling glitter. Others include impalement upon candy canes made from warp materialized sugar.
Most stories revolving around the warp demon known as Santa Claus share a similar consistency of him punishing the wicked with demented and sociopathic acts of cruelty while providing charity to the innocent. Apparently, Santa Claus even has a somewhat odd fascination with children granting sweets to those who have been good and the livers of their parents to those who were bad upon his token holiday following a planets full rotation. It's frankly an odd clash of extremes which leaves many Inquisitors baffled as to the Greater Demons motives.
On several worlds, Santa Claus, has even encouraged the systematic act of giving gifts on his token holiday of appearance. Granted, just to be clear, when you review the planetary records pretty much always this trivial holiday custom first started with a severed head stuffed into a box. But, that is besides the point. The important part is that Santa only reserves his cruelty for the wicked and not the innocent. Therefore, it should come as no surprise that while proper celebrations of Santa Claus as a modern Imperial Saint are discouraged… most institutions within the Administratum do cut operation hours on planets which observe his customs during ritual days. Coincidently, the Ecclesiarchy also tends to have record numbers of attendees to their masses during those same days of local custom. On some worlds, there are even rumors of Arbites reducing judgement sentencings to minimum as opposed to maximum at the end of court proceedings when those proceedings fall upon Santa Claus's holiday.
Now then, how does one defeat Santa Claus. After performing a comprehensive review of our historical records, we deduced a few things which might prove helpful. According to ancient Terran folklore from The First Empire, Santa Claus was once crucified and the usage of nails stabbed into his flesh proved capable of binding him temporarily into place. We, within the Inquisition, have deduced that a similar course of action might yield the same results. Therefore, we recommend attempting to stab him and pin him into place… ideally with spears.
Santa Claus has also often shown a bizarre and uncontrollable case of kleptomania for all things sweet. Field testing done by Harlequins has proven that when confronted by delicacies of sweets such as cookies and cakes, Santa will uncontrollably venture off task to devour everything in sight. If confronted by Santa Claus, and in need of a distraction, we recommend taunting him with something sweet until a better plan can be concocted. Alternatively, it might be possible to prevent Santa Claus from being reborn locally by killing off any and all examples of the local rabbit population… which is weird because Logis Fraybert insists that rabbits do not lay eggs.
As an odd side note I want to mention that despite knowing the original name of Santa Claus as Saint Christof, using his name in any attempt to banish the demon appears to have no effect. Evidently, the only way to effectively see the demon known as Santa away from the Materium is to simply wait out his holiday or attempt to slay him in personal combat. That having been said, slaying him once is only a temporary solution at best since he reappears every year upon his day of choice. We within the Inquisition believe that this might have something to do with the fear he inspires from the wicked worthy of his wraith.
Santa Clause also only appears on worlds which adhere to his traditions and know of his legend from interactions with other off-worlders. Therefore, it might also be possible to slowly eradicate the demon's ability to travel into the Materium by isolating worlds which know of him and slowly weeding them out of existence until the demon at last losses his powers.