Gray Merchant was reprinted in Theros Beyond Death along with the incredible Nightmare Shepherd, so Mono Black Devotion is an obvious place to start building.
Mono Black Devotion with Theros Beyond Death
2 Midnight Reaper 4 Ayara, First of Locthwain 3 Bolas's Citadel 4 Gray Merchant of Asphodel 4 Nightmare Shepherd 4 Gutterbones 4 Burglar Rat 2 Drag to the Underworld 4 Murderous Rider/Swift End 4 Yarok's Fenlurker 4 Castle Locthwain 20 Swamp (339) 1 Erebos, Bleak-Hearted
Arena-Formatted Decklist - Mono Black Devotion 20 Swamp (ANA) 63
2 Midnight Reaper (GRN) 77
4 Ayara, First of Locthwain (ELD) 75
3 Bolas's Citadel (WAR) 79
4 Gray Merchant of Asphodel (THB) 99
4 Nightmare Shepherd (THB) 108
4 Gutterbones (RNA) 76
4 Castle Locthwain (ELD) 241
4 Burglar Rat (GRN) 64
2 Drag to the Underworld (THB) 89
4 Murderous Rider (ELD) 97
4 Yarok's Fenlurker (M20) 123
1 Erebos, Bleak-Hearted (THB) 93
20 Swamp (ANA) 63
The best place to start is with the awesome power of Gray Merchant and Bolas's Citadel. Bolas's Citadel turns this typically midrange strategy into a combo deck. If you ever untap with Bolas's Citadel (or play it with your land drop available), you have an incredibly high chance of just winning right on the spot. The legendary artifact itself provides three devotion, and you have several options for redraws that let you keep playing spells and gaining life. Even if you don't win on the spot, you're likely to have amassed a board that can win easily on the following turn.
Nightmare Shepherd further increases the combo potential of this deck by turning any sacrifice outlet into a second copy of all of our powerful enters the battlefield effects. If you can ever get a second Gray Merchant trigger you usually just win the game. It also happens to be a 4/4 flying creature for 4 which is not terrible on its own, and provides two devotion.
There is an abundance of wealth at the 3-drop slot for this deck. Ayara provides an awesome three devotion and contributes to the general strategy of draining out our opponent while remaining at a high life total for Bolas's Citadel. Murderous Rider is a great card to have access to since it allows us to play premium removal and maintain a card with high devotion count, and Midnight Reaper helps all the pieces come together. Midnight Reaper is much stronger in the Cauldron Familiar / Witch's Oven build of this deck that we were playing in the video at the end of this article, so we've trimmed a few copies in this list and it's possible that they could be removed entirely.
With this build, we've opted for the discard 2-drops. Gutterbones is our best option at one mana since it can get in some damage early and become a recursive creature with Ayara. Yarok's Fenlurker and Burglar Rat contribute to our plan of value creatures with enters the battlefield effects, working well with Nightmare Shepherd and sticking around for devotion. In the video below we were trying Priest of Forgotten Gods as our two-drop of choice, but despite its high power level we were lacking the consistency to use it since we're focused more on the high end devotion payoffs than small creatures. Tymaret, Chosen from Death is another strong option to consider in some number, especially if the numerous graveyard synergies now available in Standard turn out to be heavily played.
We round out the deck with a few copies of the other black devotion cards. Drag to the Underworld is an extremely efficient removal spell, though it might get trimmed if we decide our deck is lean enough, since we want as many slots as possible to go towards cards that can increase our devotion. We haven't had many games where we've gotten to test Erebos, Bleak-Hearted, and while we are hurting for space on expensive cards already, it provides a little bit of every type of effect that we want.