Yaarel
🇮🇱 🇺🇦 He-Mage
You are characterizing Arioch as if omnipotent. That is neither true for the novels, nor useful for D&D gaming. There are effective limitations in play.You can't translate some parts of Elric, but not other parts and have a D&D translation of Elric. Either you translate them all, and the PC has access to a god that can act directly in the world, or you don't have an Elric translation, but instead an off brand copy. Elric is Corn Flakes, and your D&D translation is Flat Corn Pieces that don't taste as good.
The translation into D&D emphasizes the narrative limitations that exist in the novels. The concept of conflicts with other cosmic forces, bargains with Humans, and designating some Humans as champions to further the alignment cause, are some of these limitations that restrict Arioch from the influence over the rest of the multiverse that exists beyond the Astral dominion that Arioch inhabits.