The Ravenloft campaign setting states that clerics of the Eternal Order "worship a patchwork pantheon of death gods borrowed from other religions."
I don't believe it's ever been more specific than that. I would assume any deity, outlander or native, that is a death god could be a part of that pantheon, but you would have to pick the alighment and domains of the Eternal Order (NE, and Death, Evil, and Knowledge).