I was thinking the Dark/Astral culture would be "Occult". Though could be neat for them to be Shadow (base affinity) and the other affinity based on their chosen Endalion's affinity and along with other perks, like you said.I’m still thinking it would be a good idea for the expansion to add a “Cultist” culture.
Working off the lore, and the precedent set by the Primal culture’s spirit animals, the Cultists would worship one of the Endalion (Eldritch embodiments of magic, for those not in the know), which determines their affinity, and a few abilities.