Hyper efficient creatures with low casting costs and massively high strength and toughness (relative to their casting cost). This deck is both thematic and to the point. I could have included more direct damage/drain spells, but early on I decided I wanted my creatures to do the damage and my sorceries, instants and enchantments were there to make them even more efficient and threatening.
Creatures:
Carnophage & Sangrophage are strong early round hitters, and if I don't want to pay their upkeep, I can leave them tapped and use them as sacrificial fodder for other creatures. They are the only zombies in the deck.
Delraich & Hidden Horror are the only horrors in the deck that aren't also phyrexians. They are both heavy hitters.
Devouring Strossus, Phyrexian Negator and Phyrexian Obliterator are all heavy hitting black, horror, Phyrexians, and Phyrexian Gargantua is a heavy hitting artifact, horror, phyrexian. The Phyrexian Totem can transform into a heavy hitting black, horror, artifact, Phyrexian.
Phyrexian Colossus (golem), Dreadnought (dreadnought) and Walker (construct) are all artifact Phyrexians as well as the type listed. The Walker is sacrificial, the others are heavy hitters.
ARTIFACTS: The Phyrexian Totem is a good mana source as well as sneaky attacker; Gauntlet of Power makes all my black creatures better (same as Bad Moon) but also makes all my swamps super productive (double bonus!); Whip of Erebos is a critical card to the decks success, as it gives all my creatures LIFELINK...which helps offset the huge life upkeep of both creatures, spell casting and Phyrexian Arena.
SORCERIES: Demonic Tutor & Diabolic Intent will get me Whip of Erebos (Artifcat-Enchantment) as soon as possible, but can otherwise be used to dig up whatever powerful creature or nasty enchantment can give me the best edge at the time they are cast; Reprocess allows me to pitch less useful cards from my hand to draw an equal number of hopefully more useful ones; Yawgmoth's Will is the late game gamechanger that lets me have one giant devastating turn (with plenty of dark rituals and low cost creatures and spells littering the graveyard); and Beacon of Unrest is a pretty efficient way to get my power creatures back from the grave and into play...or a great way to heist opponent's powerful creatures from their graveyard into play.
INSTANTS: four Dark Ritual cards...standard fair for any classic mono-black deck.
ENCHANTMENTS: Recurring Nightmare is one of my all-time favorite cards I have been using for over 20 years, the efficiency to trade a sacrificial creature in play (weather lower strength or a tapped Colossus or a pacified horror) for something better and scarier in my graveyard is awesome, plus Recurring Nightmare returns to my hand to be used again and again and again...much to my opponent's despair (just watch out for counterspells); Phyrexian Arena (I thought using 3-4 would be too painful for me and my opponent) are here to replenish my hand; Bad Moons are here to beef up my already beefed up creatures; No Mercy is my sole defensive enchantment; and Cover of Darkness makes my horde of Phyrexians or Horrors (based on the status of the battlefield) even more terrifying by giving them all Fear.
SIDEBOARD - The four Syphon Soul cards are there as cards I would need if playing multi-player, since the ability to drain multiple opponents is super efficient and would be needed if I am to defend myself from multiple opponents PLUS deal with my life loss upkeep. The Gravecrawlers make for very efficient sacrifices, but don't bring a punch, unlike the rest of my creatures (save the Walkers). Phyrexian Tower is there only because I don't own it, but I'm thinking about buying one, if so, it'll probably get into the deck, taking one of my swamps out...I mean, it's Phyrexian...so got to have it, right?