Mono-Black devotion is a pretty simple idea, but establishing the right combos takes some work. So, here is what I've got.
Gravecrawler, he is a simple and straightforward attacker who never goes away in this deck archetype.
Nether Traitor, this guy does your early game work. He is unblockable unless your enemy has Shadow, which honestly is unlikely in Modern these days. He is also similar to Gravecrawler, he never truly goes away unless he is exiled. Plus, he has haste. Gifted Aetherborn is also in at the two spot, I believe the two cost creatures are probably the most fundamental in mid-game, and this one gives you the perks of Lifelink and Deathtouch.
Geralf's Messenger
, in case you haven't noticed yet, these creatures are all hard to kill. This one offers undying, meaning he will stick around. He also has a built in burn for two when entering the battlefield.
Phyrexian Obliterator, the ultimate black devotion heavy hitter. Again, this is a creature your opponent will hesitate to defend against or burn.
Divinity of Pride
, this is the game changer and can be played on curve in order or brought out early with Nykthos, Shrine of Nyx based on your devotion. Flying, Lifelink, and extra strength based on your life total, which the deck gives you several opportunities to gain the life needed.
Gray Merchant of Asphodel, this guy deals damage to your opponent based on your devotion to black, which is probably going to be significant by the time he is played. He can be played on curve or early with mana combos. He also gains life for your devotion to black.
Grave Pact, this card makes the battlefield a scary place for your opponent. On turn four give or take you can opt to play this card, which depends on your mana situation. Some combos, you can play this after
Geralf's Messenger
and use a kill spell like Hero's Downfall on your own Geralf to force an opponent to sac their creature and in turn making Geralf stronger for the attack phase.
Hero's Downfall, there are plenty of cheaper kill spells in Modern, that's true, but this one gives you the ability to take care of pesky Planeswalkers as well.
Thoughtseize and
Despise
both control your opponents hand. Once again,
Despise
lets you clean up Planeswalkers from your opponents hand or a creature.
Torment of Hailfire, this is a brand new card from Hour of Devastation that fits perfectly into Black devotion decks. Ramp up its power with Nykthos, Shrine of Nyx and blast your opponent to smithereens.
Erebos, God of the Dead, here is where you can score a quick heavy hitting multi-purpose creature in black devotion decks. He fits right in because he is hard to kill, he is indestructible, but his true purpose is that he stops opponents from gaining life.
Vesuva can become any land, which pairs well with Nykthos, Shrine of Nyx.
Urborg, Tomb of Yawgmoth
can also help Vesuva become useful early as a swamp if there's no other options.