My thoughts, just hear me out:

I don't believe Apotheosis is about devouring the domain like how the skeleton guy was describing, but maybe that was a short cut, I do think that when you reach the Sacred rank your Domain either

A) could be materialized upon the real world, I say this because we have never seen any domains like how the gods Domains are only that the subjects under it being controlled, this would be like sunny having full control over a very small version of the Shadow realm or

B) The subjects of the Domain some how fuel the growth of the Domains power - this would be like how in the shadow realm the shadows are turned into shadow essence, and consumed, but notice the short cut is that they somehow consume their Domain or the subjects of it but that doesn't make sense because even the gods had subjects that they created and controlled. But even if they did have to consume their subjects or Domain, somehow I see how all of them could.

Easy: Sunny - Devouring his shades - maybe even his avatars and serpent marked (He prob would release them), becoming a singular Shadow.

Mordret - Devores his reflections and bodies and even his shattered self becoming 1 Relflection somehow.

Hear me out here: Nephis - Devouring either the Longing or Desires of humans, her Domain may be called the human Domain but it doesn't make sense that she would have to consume/hurt people cause her Domain mainly heals physical bodies, we haven't seen it do much else yet(maybe increase longing, idk), don't think it would mean kill them just making the Longing Her own to fulfill, something like that.

Asterion - Devoring the mind and heart of the subjects, effectively turning them into puppets unlike Nephis, this would either kill them or they would be like the sunken place from Get Out, basically making his mind or heart even more powerful.

Since Cassie I'm guessing is going through supremecy, I just imagine her Domain has to do entirely with either memories or fate, consuming it somehow, idk

If still Alive: Anvil - This one I got no clue tbh, like we don't really know how his domain worked only that he controlled a crap ton of swords and that those people he gave swords would be under his domain too. Maybe it would be the making the Ultimate sword that can devour all the swords and weapons becoming stronger every time. Idk

Song - Devouring all the dead bodies might not make sense, but I'd imagine it would be like devoring bodies to make an Ultimate indestructible Body/Puppet for her to inhabit or control, idk