Athreos, God of Passage is a card that I am really interested in building around. I think there are two fundamental ways to approach Athreos for a competitive list. First avenue is voltron strategy where you get your devotion count up quickly and start swinging. The second one is a abusing the 3 life recursion aspect; this is what I want to focus on. How do you make the deck in a manner that the 3 life matters? I don't see many situations where the opponent would prefer the creature to be in your hand vs. in your yard nor situations where 3 life is worse than returning the creature.
In a 4 player game, you would need to return 40 creatures to kill everyone based on Athreos's ability. Most games I have seen, people go through 30 cards of their deck, which is about 7 creatures. B & W are not strong draw colors and as such, could mean less digging through your deck.