Welcome, and thanks for taking a moment to check my deck!
While its core is that of a classic Mono Black Devotion (as it actually was some time ago), the addition of red gives to this new take a lot more of speed and stability. This deck is tons of fun to play and definitely a competitive one.
Now, let's crack the cards a bit!
Creatures
Gatekeeper of Malakir: Solid drop, it gives us a creature, takes one from the opponent, and contributes for 2 devotion.
Geralf's Messenger: This dude is a powerhouse. Nice ETB effect, recursion, resilient against most removal (except exile), 3 devotion, all packed in a single card.
Gray Merchant of Asphodel: Our fatality. No, seriously. At its worst, it's a 2/4 that drains 2 life. At its best, it wins games on the spot.
Relentless Dead: One of the most recent additions to the deck, it's a great utility card relevant in all the stages of the game. It's an evasive beater, an excellent blocker, and a tool to recur creatures from the graveyard. And all this comes with self-recursion and 2 devotion!
Stillmoon Cavalier
: One of our finishers, it's a great mana sink that is almost immune to removal (and easily recurrable in this deck in case the opponent is playing red). It will give your opponents more than one headache while they're trying to deal with it.
Vampire Nighthawk: A 2/3 evasive, life-gaining, creature-killing body. Just, wow.
Noncreatures
Duress: Specific discard against some threats. It can easily take off combo pieces.
Inquisition of Kozilek: Our main form of discard, it targets everything this deck hates. Quick creatures, combo enablers, removal ... It never misses.
Kolaghan's Command: This card screams "utility" and "2x1". Great tool that will never be dead in your hand.
Lightning Bolt: We play red, we play 4 Bolts. Not much to say here.
Sign in Blood: Digging through the deck is important, and this does the job efficiently. Can work as an emergency Shock to kill the opponent.
Terminate: One of the best removals in Modern (if not the best), it hits almost everything.
Lands
Bloodstained Mire: Fetch lands are always useful to find the colors you need and thin your deck.
Blood Crypt
: The most efficient, fetchable dual lands.
Ghost Quarter: Great land hate, it takes care of manlands and other things you don't want around. You can even crack tapped lands for an additional mana if you're really desperate.
Mountain: Basic mountains aren't really necessary, and we're packing a single one just for the rare case we need to fetch red for only 1 life.
Nykthos, Shrine to Nyx: More mana you will know what to make of in late game.
Swamp: Duh. We're playing Black Devotion.
Urborg, Tomb of Yawgmoth: It helps us fixing our mana base. We need black after all.
Sideboard
Crumble to Dust: I heard you have a Tron problem.
Duress: When you need more targeted discard, board in the full set of these boys!
Elixir of Immortality: Used mainly against Burn, but will help against the occasional rogue Mill or to stabilize better in some games.
Leyline of the Void: When graveyard hate is needed, this is the best solution; it provides devotion on turn 0 too!
Pithing Needle: Takes care of planeswalkers and tons of other things.
Pyroclasm: An efficient board wipe against fast aggro decks. Our own creatures don't die to this, or don't mind dying at all.
Rakdos Charm: Great utility card with three equally interesting and relevant modes. Nice tech to have.
Slaughter Games: Most combo decks concede to this.
Sudden Death
: Uncounterable removal. Surprisingly useful in a lot of situations.
And that's all. Upvote if you like, and comment if you want! Any suggestion is welcome!