Of all the characters listed here, this one is the least likely to appear in the series. This is largely a guess on my part, but my other option was to cast Charles Dance as the Emperor of Nilfgaard, which the games already did. At any rate, Eredin is the King of the Wild Hunt, an ethereal being who leads a horde of spectral riders in between worlds. They're a thoroughly ghastly presence that most write off as myth.
Keith David is one of the most prolific voice actors currently working, and it's easy to see why. His voice is deep, rich, and can be both comforting and deeply intimidating. While Eredin is an elf, the role of this entity is likely going to be voice-only, which would make Keith David perfect for the part. Even if it's only a fleeting appearance, this figure, combined with this actor's rich bass tones, could make an incredible impression.
Self-evidently a man who writes for the Internet, Robert also writes films, plays, teleplays, and short stories when he's not working on a movie set somewhere. He lives somewhere behind the Hollywood sign.