Mono-Black is one of the better choices for a deck in Magic The Gathering's Commander format. They have access to a lot of powerful rituals to help them facilitate some ramp, as well as access to a plethora of tutors to help them get just about any card they need at any given time.There are a lot of different ways to build a Mono-Black deck, from aristocrat to lifegain. Mono-Black has access to a lot of cards that benefit from multiple swamps being controlled, and since you're in a Mono-colored deck, nearly all your lands will be just that.
10 Feed The Swarm
Feed The Swarm isn't exactly an amazing card, but a necessity in any Mono-Black deck. Feed The Swarm is the only card Mono-Black in the entire game that can destroy an enchantment.
In Commander, it's not uncommon for the table to be filled with powerful enchantments, so it's important to always have a way to remove them. Tutors help you to get to it when you're desperate for removal. Feed The Swarm is also beneficial in that if there are no enchantments, it can be played as a simple creature removal spell.
9 Gray Merchant Of Asphodel
Gray Merchant Of Asphodel, or Gary as the community has nicknamed it, is a perfect fit into a Mono-Black Commander deck. Since it focuses on devotion to Black, odds are you'll be getting its effect off for high amounts.
When Gray Merchant hits the field, you gain life while each opponent loses life equal to your devotion to Black. It does cost five mana to cast, but high-mana creatures aren't an issue for Mono-Black, especially one that helps you with a bit of a comeback factor in terms of life.
8 Crypt Ghast
Crypt Ghast is a tool that helps Mono-Black to ramp into their big spells easier. It has a passive that makes every swamp tap for an additional Black mana. It also has extort, which lets you tap either for White or Black to make each opponent lose a life and gain that much life.
Doubling up on your mana is very good, especially in Mono-Black where its best cards cost a lot of mana. Even if you don't have an extra spell to cast, you can simply use the extra mana for its extort effect instead.
7 Cabal Coffers
Cabal Coffers is one of the best lands you can run in a Mono-Black deck. By paying two mana and tapping it, you add a Black mana equal to the number of swamps you control. Since most of your lands will be swamps, you'll be adding a lot of mana to your pool.
Cabal Coffers is one of the easiest ways to cast your high-mana spells, which Mono-Black is no stranger to. There are a lot of very powerful creatures that cost a lot to cast, and Cabal Coffers makes casting them almost trivial.
6 Dictate Of Erebos
Dictate Of Erebos is a fantastic Mono-Black enchantment that makes everyone think twice about killing your creatures. Whenever a creature you control dies, each opponent has to sacrifice a creature. It has flash too, making it easy to cast during end steps or to catch opponents off guard.
Mono-Black can easily get a sacrifice engine going, which will constantly trigger Dictate Of Erebos. Forcing sacrifice is the best kind of removal in Magic The Gathering, so being able to do it easily while not being once per turn is phenomenal.
5 Mutilate
Toxic Deluge is one of the best board wipes in the entire Commander format, but in Mono-Black decks, they can essentially run two copies of it. Mutilate is another board wipe that gives all creatures -1/-1 for every swamp you control.
Since you're in Mono-Black, most of your lands will be swamps and so the toughness that creatures lose is often enough to kill them. It's not as powerful as Toxic Deluge, but with no cost outside of its mana value, it's a fantastic include as a second board wipe for Mono-Black Commander decks.
4 Black Market
Black Market is a way to easily ramp in Mono-Black. Whenever a creature you control dies, Black Market gets a counter on it. Then, during the start of your precombat main phase, you add Black mana to your mana pool equal to the counters on it. This doesn't remove the counters, allowing them to stack up quickly.
Since aristocrat is one of the more popular Mono-Black builds, Black Market will get out of hand very quickly with mana. It makes casting all the spells in your hand easy, allowing you to keep mana from lands up for removal and interaction.
3 Bolas's Citadel
Bolas's Citadel is a six mana legendary artifact that gives you a ton of versatility. It lets you look at the top card of your library and play them at any time. If it's a nonland card, you can pay life to cast it rather than paying mana. It can also tap itself to sacrifice ten permanents to deal ten damage to each opponent.
It's easy for Mono-Black decks to flood the field, especially in aristocrat builds. The ability to know what your next draws are, and cast them while saving mana pushes Bolas's Citadel over the edge in terms of how good it is.
2 Vilis, Broker Of Blood
Vilis, Broker Of Blood is one of the best Mono-Black cards in Commander. It cost a lot of mana to cast, but when it hits the field you have both a removal and a draw engine stacked into one card.
By paying a Black mana and one life, you can make a creature -1/-1. You can continuously use this to kill the creature. This will also draw you a card with each activation, as whenever you lose life, Vilis will let you draw that many cards. This doesn't just trigger from its own effect, but any form of damage. So if you're ever attacked, you'll be drawing a lot of cards less they kill you in one swing.
1 K'rrik, Son Of Yawgmoth
K'rrik, Son Of Yawgmoth is not just a fantastic Mono-Black Commander, but a phenomenal card to include in the 99. Since you're only in one color, if K'rrik is on the field, you'll never have to pay mana for any Black mana needed to cast a card. Instead, you can pay two life.
Whenever a Black spell is cast, K'rrik will get a +1/+1 counter, and since most of your spells will be Black, K'rrik will start growing early and quickly. Black can easily gain life as well, making the life cost substitution trivial and sometimes even allows you to gain mana rather than losing it.