@Vengeance of
Apparently yes, souls contain shadow anima in the form of memories of matter, but not the matter itself, and the Underworld is probably formed of shadow anima instead of anima:
"My thesis is that this Underworld is formed not of matter, but of the memories of matter, of the detritus left behind by the souls that have passed on, like a snake shedding its skin. "
- The Magister's Journal 1
https://runescape.wiki/w/The_Magister%27s_Journal_1
The major difference between Amascut and Itchlarin is Amacut feasts on the corrupted souls and they made her corrupted and weak, but Itchlarin guards the souls and apparently gained their powers and at least certain immunity to corruption:
"There is one, Amascut, named now as the Devourer, who has left her mark on the very essence of the people here. A foul corruption has spread across their bodies and threatens their souls.
It is not a useful ailment. It damages the souls, leaves them weak, vulnerable, and ultimately unfit for my experiments. A pity. Fascinating, but ultimately problematic.
Another, more active, deity is the one I have encountered many times on my journey back to the Underworld: the dog-like Icthlarin.
An interesting sort, obsessed with guarding the souls on their journey to an afterlife. He seems burdened by an ancient guilt; perhaps that is why I feel a sense of kinship with him?
I wonder if he realises the sheer power at his disposal.
If he just wanted... If he just reached out and used the vast energy flowing through his domain on a constant basis, he could rival any of the other so-called gods."
https://runescape.wiki/w/The_Magister%27s_Journal_3