Would this extend to the Eternal Cabal being compatible with Matronae, as in both can exist in the same city without conflict? The Eternal Cabal sounds intriguing regardless.
I'm not sure about the faiths coexisting peacefully, and am less sure now about not giving them Divine.
I tend to think of the in game Eternal Cabal as representing all of the faiths centered around the god of death, even when that god of death is not Arawn.
One of the main groups to oppose the Matronae are the cult of The Haunted, who worship Laroth as the new god of death and seek to aid his deification. Laroth and his chief lieutenant Arak the Erkling do not get along with Gyra/Morrigan well at all. Laroth tries to use Auric Ulvin to actually absorb the full power of Arawn and become the true god of death, but Auric ends up taking over the position of God of Death for himself. (It would violate the compact for a mortal to have more power than a god so he could not claim both the spheres of Ice and Death at the same time, but the Ice sphere he had previously claimed seemed to have left him when he died by the Netherblade.) A lot of those who worshiped Arawn accept Auric as their new god. Whereas Arawn had despised The Betrothed and cursed them to become the undying Silkies, Auric accepted them as his brides.
Do they have a temple beyond the Sidar shrine?
The Reliquary is the Eternal Cabal temple now, instead of an Elohim UB.
The Elohim can still get the Reliquary more easily than other civs, as their Devouts can be sacrificed to Force the Construction of the building ignoring all prereqs like the religion being present in the city already. The Sidar usually found the Eternal Cabal first when they build an early game Shrine of Arawn, but if they are not in the game then the Elohim tend to found it instead.
The Eternal Cabal is canonically a very popular faith among the Elohim, although perhaps not as popular as it was with them during the Age of Magic. The Betrothed were a splinter sect of the Eternal Cabal found only among the Elohim, so Maoliosa the Barren, Neasa, and all of the Silkies were Elohim women before they committed ritual suicide in the belief that Arawn would be pleased by such a sacrifice instead of abhorring it.
The Elohim city of Niveus also becomes the center of the Matronae faith, as it there that Morrigan (formerly Gyra) forms the Sepulchur of the Matronae. I imagine that many of those who were in the Eternal Cabal would shift to worshiped Gyra/Morrigan.
The Eternal Cabal's religious champion is the Silkie, which is naturally immortal. (Thanks to the last MNAI update they at least don;t get reborn after abandoning you for switching religions.)
They have 3 choices of naturally immortal heroes, depending on the alignment.
if you are Evil you can train Maolisa the Barren, who has Vile Touch and Marksman.
If you are neutral you may train Khalida the Suicidal, who looses strength when she kills but becomes much more powerful every time she dies and is reborn.
If you are Good you can train Neasa. She had Guardsman and Arawn Death Affinity, but not much strength. Canonically she is the youngest and friendliest Silkie, whom Maolisa had tricked into killing herself before reaching puberty. While the other Silkies follow Auric once he becomes God of Death, Neasa instead stays in the Elohim city of Niveus and is often seen near Morrigan's Sepulchur of the Matronae.
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I'm thinking now that I may leave Graft Flesh as the Carnifex (Annointed Priest) spell while making a new "Create Chimera" spell for Body Affinity + Channeling 4. That would create a unit randomly from among those creatures which were first created by Patrian mages. These include Manticores, Griffins, Wyverns, Hill Giants, Colubras, Centaurs (and their upgrades), Lizardmen (and their upgrades), Lamias (adepts or mages, as letting you summon limitless archmages would be too much), Mustevals (and their upgrades), Minotaurs, etc. These would not be limited duration units, so I'm not sure about whether they should get the caster's summon perks.
The Bestiary implies that these were creation of the Life sphere made by the students of Leucetios, but Kael on reddit did specifically say that "Kezef mastered the art of being able to stitch various creatures together, he created the chimera and the flesh golem." I still tend to think that Life magic may be needed to make such hybrid beings breed true with each other and become enduring new races, but maybe that isn't needed just to make a monstrous servant. I'm thinking Kael made have done a little retconning honestly, as earlier sources said that the perversions of Life were the most vile of all the crimes the Patrian mages, far worse than necromancy, but more recently Kael claimed that Leucetios was a good person and the only member of the Imprimatur council to ally herself with the repentant Kylorin instead of maintaining the corrupt order along with her peers.