Pressuring lifetotal is the key here. Athreos and Mogis seem to have something in common… (yes, black).
Athreos’s Two-Colored Cards
[scryfall]Ayli, Eternal Pilgrim[/scryfall] seems pretty good here. Its highly efficient, but the synergy with Athreos seems very good. Maybe [scryfall]Maw of the Obzedat[/scryfall] is a less oppressive choice.
[scryfall]Cliffhaven Vampire[/scryfall] and [scryfall]Drana’s Emissary[/scryfall] are both recent cards that would be very irritating in this deck. Actually, all cards would be very irritating in this deck. They are powerful, but maybe not too bad in this environment.
[scryfall]Kingpin’s Pet[/scryfall] and [scryfall]Tithe Drinker[/scryfall] are one of the better cards with Extort.
[scryfall]One Thousand Lashes[/scryfall] is a pretty interesting card here, but maybe I would rather go with [scryfall]Pillory of the Sleepless[/scryfall]. This does present a problem, because I wouldn’t want to put strictly worse cards into the cube.
[scryfall]Orzhov Guildmage[/scryfall] and [scryfall]Scholar of Athreos[/scryfall] do very similar things, but I guess I’ll go with the Guildmage, because its less oppressing.
[scryfall]Sin Collector[/scryfall] could be a nice addition. Its not too powerful in the cube, because there aren’t going to be that many spells, but at the same time, sometimes you want to protect your bombs.
[scryfall]Souls of the Faultless[/scryfall] would be good for the Devotion, but otherwise I think its a possible addition. Also, if the Phenax-player wants to splash this, its a very powerful choice, and I like that kind of competition.
What we want here is some good, aggressive creatures, that you can recast often.
[scryfall]Mardu Woe-Reaper[/scryfall], [scryfall]Soldier of the Pantheon[/scryfall] and [scryfall]Dragon Hunter[/scryfall] are nice, strong [scryfall]Savannah Lions[/scryfall] with upsides that can sometimes be important in this format. I don’t like how swingy Dragon Hunters abilities can be, but its going get in anyhow.
[scryfall]Ajani’s Pridemate[/scryfall] seems powerful enough to bring along. Other two drops that seem interesting include [scryfall]Herald of Dromoka[/scryfall] (although I’d have to check how many Warriors there actually are in the cube), [scryfall]Kami of Ancient Law[/scryfall] and its variants, [scryfall]Master of Pearls[/scryfall], [scryfall]Oreskos Sun Guide[/scryfall], [scryfall]Seeker of the Way[/scryfall], [scryfall]Thraben Heretic[/scryfall], [scryfall]Topan Freeblade[/scryfall], [scryfall]Veteran Armorer[/scryfall] and [scryfall]Whitemane Lion[/scryfall].
There are plenty more good two-drops and aggressive creatures, but I’ll fix the curve later. For now, the selection is this:
Black’s approach to these things is quite different and generally its creatures are weaker, but have interesting abilities. I don’t want to put creatures into the cube that are just strictly worse than their white counterparts (well, disregarding color, of course), so I’d rather find creatures with drawbacks, that can do something the white creatures can’t.
Also, [scryfall]Carrion Feeder[/scryfall], which can have pretty good synergy with Athreos.
I can’t even justify all these choices, but there they are.