There's a bit of explanation for why I'm about to say what I'm about to say, and I'll put most of the academic-theological material in a spoilerbox for easy ignoring by any not that interested ...
... but one of the best models for the conceptualization and operation of the Custodes is actually that of the Huscarla of a Nordic lord.
I'll quote from an article I wrote last year:
Nevertheless, there is a significant resonance to be found in all three of these definitions, properly construed, with the Nordic concept of the Huscarla – the House[‘s] Karls [‘Men of the House’]. A term which, despite its modernly familiar meaning of a body of armed men in the personal retinue of a lord, had an older swathe and broader of meaning which also encompassed other, not-immediately/directly-militarized free-men of the Lord’s House. Interestingly, another term – Hird – went in the other direction over a similar period, from meaning just such an armed retinue of personal guard, to a more generalized concept of a ‘household’, and even a royal court.
[there is also a further element I did not include there, around the spies of such a court going out among the broader populace - 'Geist' being phonetically close to both "Ghost" and "Guest" is *entirely* unaccidental]
And, for the preceding few paragraphs ..
Now as for why this is relevant ... it isn't just because the Custodes are something almost akin to 'family' to the Emperor - constant companions, regarded with genuine affection, and certainly definite 'familiarity' ; nor is it because there is a general typology of a feudal iron age lord with a retinue that The Emperor is quite literally the apotheosis of the concept of.
Rather, it's because there is a *quite specific* Indo-European mythic occurrence that The Emperor and His Custodes are the Far Future manifestation of.
"In any case, both of these Old Norse concepts align most strongly with the sense of the BhutaGana, in its aforementioned broader sense, as the Retinue, the Chamber Militant, indeed, the *Companions* [and I mean that term, too, also in its far older militaristic sense] of Lord Shiva. Although it is perhaps worth noting that while both are very much in accord with the concept of the Einherjar, neither *quite* encapsulates the severe degree of closeness with which the Einherjar were occasionally regarded in relation to Odin – stated in the Gylfaginning to be as that of His adoptive Sons.
This notion of “Sons” (adoptive or otherwise) is one which comes through repeatedly when detailing the Ganas of Mahadev. As we have already heard, the Rudras and the Maruts may be direct instances of the Sons of Shiva, arrayed about Their Lord in resplendent, Roudran (that is to say – ‘Roaring’) might."
It helps, of course, that the deity I am referring to is, in the Hindu perspective, headquartered in the Himalays, referred to as wearing golden armour, accompanied by a formidable guard of lightning-ensigned super-warriors wielding spears/lances, at least some of whom fly on roaring 'horseless chariots' [which, for obvious reasons, I keep thinking of as jetbikes], etc. etc. etc. Oh, and "Ishvara" also translates rather handily to "God-Emperor".
Now part of the reason I bring all of this up - other than the fact that it's pretty strongly resonant in conception with the Custodes [seriously, there is a *reason* that article I wrote includes headings like "The Gana As Custodian Guard" and "The Golden Legion of Thunder Warriors" in it] - is because there is a particular mythological instance that really strongly gets across how I see the Custodes working, particularly during Unification.
That is the Vayu Purana account of VeeraBhadra at the Horse-Sacrifice of Daksha.
The long and the short of which is, there is a ruler on Earth who is carrying out a sacrificial rite which, if completed, shall seal/signify his status as the over-king, the unchallenged paramount sovereign of ... pretty much everywhere; and, alongside this, is also honouring a deity as being supreme.
Daksha is warned by the sage Dadhichi that this is ... going to end badly - "The man who worships what ought not to be worshipped, or pays not reverence where veneration is due, is guilty, most assuredly, of heinous sin.
This rather annoys Rudra [the aforementioned Himalayan-dwelling God-Emperor], Who dispatches VeeraBhadra as His Emissary to put a stop to proceedings, and implicitly to bring Daksha to heel.
You can see how this resonates rather strongly with the Emperor's general ethos on late-Unification Terra. And also, for that matter, with the role of particular Custodes as His high Emissaries, Enforcers, and potentially also sanctioning Executioners.
I won't quote the various evocative descriptors given for VeeraBhadra or the ensuing assault [although there's some pretty excellent epic verse, even in translation, to be found in the H.H. Wilson translation
] ; however it would be extraordinarily remiss of me *not* to extol the climactic verse:
"Daksha the patriarch, his sacrifice being destroyed, overcome with terror, and utterly broken in spirit, fell then upon the ground, where his head was spurned by the feet of the cruel Vírabhadra. The thirty scores of sacred divinities were all presently bound, with a band of fire, by their lion-like foe; and they all then addressed him, crying, 'Oh Rudra, have mercy upon thy servants: oh lord, dismiss thine anger.' Thus spake Brahmá and the other gods, and the patriarch Daksha; and raising their hands, they said, 'Declare, mighty being, who thou art.' Vírabhadra said, 'I am not a god, nor an Áditya; nor am I come hither for enjoyment, nor curious to behold the chiefs of the divinities: know that I am come to destroy the sacrifice of Daksha, and that I am called Vírabhadra, the issue of the wrath of Rudra. Bhadrakálí also, who has sprung from the anger of Deví, is sent here by the god of gods to destroy this rite. Take refuge, king of kings, with him who is the lord of Umá; for better is the anger of Rudra than the blessings of other gods.
Now imagine some powerful Unification-era Terran court featuring some pretend-world-conqueror who hasn't *quite* gotten the message yet, and a chosen Custodes emissary turning up in its midst in an encounter perhaps not unlike the above ... and you can see what I mean.
Particularly as applies *that* time period, 30k background really is built from Bronze Age Solutions to Future-Past Problems.
Vox Stellarum efforts:
The Unyielding Adamanticores
[truescale Astartes, auxilia, and sectorial AdMech; M37/38]
The Haunting Harii of Hvergelmir
[truescale Astartes ... and associates]
The Unification Wars
[truescale Thunder Warriors, (Proto-)Astartes, Army of Unification, Aegyptian-style Proto-Mechanicus raiders, etc.]
[Tomb Raiding for Fun & Prophet]
[truescale loyalist IVth, VIIIth; Agents of the Sigillite; VIth; and so much more; and Umbral's amazing Traitors]
The Worlds-Wide Webway
[(Dark) Eldar, Imperial Inquisition, an a-maze-ing realmweave of fear and wondermeant]
[some rather different Inquisitorial Storm Troopers, true-scale Fire Hawks, Black Dragon, Inquisitor and associates, etc.]
For Whom The Great Bell Tolls Thrice
[attempts to put the 'priest' back in 'tech-priest' with more-medieval mechanicus]
[the log that incepted it all - at least two Inquisitors, truescale Deathwatch, local enforcers, cultists, etc.]