Chaos Gods of Law
"Chaos is order yet undeciphered."
The Chaos Gods of Law are three (or more) Chaos Gods who were created in the first edition of Warhammer Fantasy Roleplay. They were referenced later in the "Apocrypha Now" supplement, and were vaguely alluded to in various Warhammer works throughout the years.
In the Black Library novels for Archaon published in 2014, Be'lakor informs Archaon that they never existed giving them their first mention in the modern Warhammer era. However the canonicity of this is dubious as there was a very distinct disconnect between Black Library and army book canon illustrated particularly well in the End Times event. Be'lakor is not a reliable source of information however as he only claimed once that he never lies apparently because it makes his schemes boring, and this was in a Warhammer 40,000 novel regardless.
If the Chaos Gods of Law ARE canon, then they would also exist in 40k as well.
All three identified ones are siblings. Solkan and Arianka are lovers. It was implied there were additional Chaos Gods of Law, but none were ever identified. Solkan, Arianka, and Alluminas seem to parallel Khorne, Tzeentch, and Nurgle but Slaanesh never had an opposite. Considering Slaanesh's disappearance in Age of Sigmar, this makes an interesting coincidence. Slaanesh's description as the youngest Chaos God also bears mentioning, as it could be possible the fourth Law god would be introduced later.
Solkan is the warrior god of Law, and thus would be Khorne's polar opposite. Witch Hunters primarily worship him and he empowers them against the Chaos Gods of...Chaos. It isn't stated if his worship is accepted within the Empire like Morr or Shallya, or if their worship is a heresy. The 40k equivalent however would undoubtedly be heretical, as just about everything is.
While Solkan represents the enforcement of law, Arianka is the disciplining nature of it and thus is a patron of warriors. Tzeentch found her to be a nuisance who could potentially unravel his designs, so he manipulated her somehow into becoming trapped inside of a coffin made of Laihtero, the rarest form of Gromril which resembles a crystal that constantly emits soft light. The keys to her prison are hidden somewhere in the mortal world. The coffin itself was hidden beneath the Empire city of Praag, capital of Kislev. Her sword Laihtendrung was also made of Laihtero.
A god of the most absolute form of light, whose gaze renders other beings into unchanging unmoving light, implying he is possibly Nurgle's opposite. He is not worshiped by many mortals because his portfolio is alien and incomprehensible entirely to them (well, technically all Chaos Gods are but Alluminas is without something simple or appealing like the others have). Note that this was before the lore that empowered the Chaos Gods by worship.
Most have regarded Empy as WH40K's equivalent of the Chaos Gods of Law and Order, since he combines the light themes of Alluminas as well as Solkan's form of authority and his opposing nature of Chaos in itself. Interestingly, multiple theories suggests that if the Emperor were to die, he would instead transform into a literal Chaos God of Order which would create a stalemate or defeat the Chaos Gods once and for all and save humanity at the same time, although this may come into conflict within certain other theories of his demise. The answer still stands that he is the only 40k analogues of the fantasy counterparts. He also embodies aspects of Malal with his status as a divine being who is a proponent of atheism and his habit of consuming Warp energy along with his status as the outsider of the group.
|The Chaos Gods of Warhammer 40,000 and Warhammer Fantasy|
|Four Main Chaos Gods:||Khorne - Nurgle - Slaanesh - Tzeentch|
|Other Gods of Chaos:||Archaon - Horned Rat - Malal - Necoho - Zuvassin|
|Chaos Gods of Law:||Alluminas - Arianka - Solkan the Avenger|