Chaos Gods of Law
"Chaos is order yet undeciphered."
- – José Saramago
The Gods of Law, sometimes referred to as the Chaos Gods of Law (Chaos in the sense of being made out of the magical element of "physics and logic can go fuck themselves" like the Chaos (AKA destruction)-aligned Chaos Gods), are three (or more) 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. The Gods of Law are separate from the other non-Chaos Gods of the world, such as those of the Empire, and seem to predate them. They may or may not be on good terms.
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 ("Trust me bro!" coming from a Daemon is never a good sign) because it makes his schemes boring, and this was in a Warhammer 40,000 novel regardless.
Four Gods of Law were said to exist, though only three were named. All three identified ones are siblings. Solkan and Arianka are lovers. The fourth was never 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. He's basically The Punisher in god-form, relentlessly pursuing and destroying any creatures of disorder or those who commit heinous crimes. Unlike his sleeping incesister and unknowable brother, Solkan seems to get shit done, at least in the old HeroQuest material, where he manifests to kick Khorne's ass when the Blood God tries to take over the Reman Empire (ancient Tilea). Witch Hunters primarily worship him and he empowers them against the Chaos Gods. It isn't stated if his worship is accepted within the Empire like Morr or Shallya, or if their worship is a heresy, though his popularity among Witch Hunters suggests it's a contended viewpoint. 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. While Arianka stopped being mentioned by name after copyright issues led to Malal being removed, a "mysterious woman" is often said to be sleeping beneath Praag, empowering ice magic for Kislev's witches.
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, though his unchanging nature yet unknowable form could put him as both an equal and opposite to Tzeentch. He is not worshiped by many mortals because his portfolio is alien and incomprehensible entirely to them (well, technically all Chaos Gods are as well 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. The latest mention of Alluminas comes from Total War: Warhammer, where Light Wizards sometimes call on him for power when casting spells.
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 being a lazy catatonic bastard like Arianka 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 Omnissiah may or may not be an avatar of him, or a separate God of Law altogether with its focus on orderly machinery. Outright stating the Emperor is a Chaos God would earn you a phosphex bath, if the enraged listeners don't beat you to death first. Either that or Khorne promotes you to Daemon Prince on the spot out of the sheer size of your balls.
The Greater Good
The only other equivalent of a "God of Law" is the Tau's coalesced idea of a Greater Good driving their actions, solidified by their highly regimented, orderly society and culture. Depending on the source they range from treating this concept as a god outright, to viewing it as naturally a part of nature, to being ardent atheists in general, but their collective belief in the concept is implied to have indeed created a godly entity. Even if they don't worship it, 40K abides by the rule of "if you believe in it, it exists" for the Gods, so it'd make sense if one formed even if the Tau don't recognize or devote a religion to it. It's a very recent addition to the fluff however, and generates skub among some Tau and non-Tau players alike.
|The Chaos Gods of Warhammer 40,000 and Warhammer Fantasy|
|Four Main Chaos Gods:||Khorne - Nurgle - Slaanesh - Tzeentch|
|Other Gods of Chaos:|| Archaon - Hashut - Horned Rat - Nuffle |
Malal - Necoho - Zuvassin
|Chaos Gods of Law:||Alluminas - Arianka - Solkan the Avenger|