It hasn’t been so long since Mono-Black Devotion was a terror that dominated Standard. The interesting thing about most decks that dominate their Standard formats is they find a new home in Modern, where they pick up new and exciting tools and interactions to keep up with the format. Mono-Black Devotion in particular is interesting because of how powerful discard spells are in Modern, combined with the existence of Phyrexian Obliterator, which may be the biggest devotion incentive ever printed. It’s unfortunate for Phyrexian Obliterator that Path to Exile and Fatal Push are the removal spells of choice, but Brian DeMars still thinks Mono-Black can be made to work:

Mono-Black Devotion - Modern | Brian DeMars

• Creatures (21)
• 2 Phyrexian Obliterator
• 3 Bloodghast
• 4 Gatekeeper of Malakir
• 4 Geralf's Messenger
• 4 Gray Merchant of Asphodel
• 4 Relentless Dead

• Planeswalkers (3)
• 3 Liliana of the Veil

• Instants (6)
• 2 Dismember
• 4 Fatal Push

• Sorceries (9)
• 2 Collective Brutality
• 3 Thoughtseize
• 4 Inquisition of Kozilek

• Lands (21)
• 15 Swamp
• 1 Urborg, Tomb of Yawgmoth
• 1 Verdant Catacombs
• 4 Marsh Flats

• Sideboard (15)
• 1 Collective Brutality
• 1 Flaying Tendrils
• 1 Night of Souls' Betrayal
• 1 Phyrexian Obliterator
• 2 Surgical Extraction
• 2 Wrench Mind
• 3 Leyline of the Void
• 4 Fulminator Mage

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Relentless Dead is the most interesting choice here. It’s an incredible creature in a format defined by Tarmogoyf and Death's Shadow, since a pair of Relentless Dead can block those creatures endlessly, forcing opponents to play into your Liliana of the Veil and more. Gatekeeper of Malakir is also a great removal spell, because it doesn’t care about the size of creatures. Gatekeeper combined with Fatal Push and Liliana of the Veil means that you have no shortage of ways to fight over your opponent’s cheap creatures.

At the top of the curve, Geralf's Messenger is a powerful threat that hasn’t seen much Modern play. When the metagame is fast, Geralf's Messenger isn’t really where you want to be. It’s a very large and resilient threat, but awfully slow for a format like Modern unless the format has shifted to favor particularly ponderous strategies. Phyrexian Obliterator is a spectacular threat if your opponent is leaning on creatures or Red removal spells, and Gray Merchant of Asphodel is still a fantastic way to steal game out from underneath your opponent.

At its core, this is a deck that is trying to play the Jund gameplan of playing efficient interaction and power, resilient threats. All told, I think it does a pretty good job. There’s always room to shift the balance of discard spells and removal spells to find space for cards that might be better or worse in your particular metagame. Things like Bitterblossom and Phyrexian Arena, Victim of Night and Duress all deserve some amount of consideration given the appropriate context in the metagame. An archetype with so many efficient, powerful, and flexible cards should always be kept in consideration.