While this list is how I personally like to run my deck, there are still a number of cards that are still very effective here. Add them however you like to better fit your playstyle. These are in no particular order.
Blood Moon - More situational depending on your opponents colors, so don't run this if you're against a lot of red already. There's plenty of basic swamps, so this shouldn't detriment you too much.
Polluted Bonds - This card is great, especially vs heavy ramp like in green. Just be sure to swap Rain of Gore for Tainted Remedy so you don't screw yourself over.
Exquisite Blood - This is a very, very strong card in black. Any time your opponents lose life from ANYWHERE, you gain life. Like with Polluted Bonds, use Tainted Remedy so you don't hurt yourself. If you really want, you can also run Sanguine Bond for the combo, but I don't find it as reliable as other options. Plus Exquisite Blood is already very powerful by itself.
Leyline of the Void - Great vs graveyard decks. If you run this, be sure to take out Bloodchief Ascension since it would make it useless. I will note that Bloodchief Ascension is usually the stronger choice in this deck, but it's up to you.
Grave Betrayal - Speaking of graveyards, this is a great way to take advantage of those who are keen on sacrificing non-token creatures for Mogis. This lets you get all of the creatures they sac onto the field under your control. This can be devastating after a board wipe, but that usually costs a ton of mana. Your best bet for a win condition is to play this when the battlefield is flooded with creatures, then play Blasphemous Act for just 1 mana. As you might expect, Bloodchief Ascension works very well with this, but Leyline of the Void doesn't.
Rakdos Charm - Great vs graveyard decks or decks with a bunch of creatures. This should absolutely be at least considered when building your deck.
Temporal Extortion - An extra turn spell? Sure! This card is pure flavor, and it's also usually a win-win since an opponent can pay half of their life to counter it and adds some neat politics to the game. Be sure that you don't have too many things to hurt yourself on the board like Havoc Festival since it'll probably hurt more than help.
Master of Cruelties - A powerful creature without a doubt. Being able to bring someone down to 1 health with Mogis on the field can easily kill someone. I have him in my deck often, but I usually focus more on enchantments. If you do run this, be sure to also run Rogue's Passage to ensure the hit.
Gray Merchant of Asphodel - Another powerful creature, and considering our black devotion, playing it can be devastating.
Stranglehold - I usually don't run this unless the meta is centered around taking extra turns, but making opponents unable to search their libraries is still very useful. Especially with the presence of tutors and fetch lands.
Zo-Zu the Punisher - Yet another way to make your foes hate playing lands. Running this with Polluted Bonds is particularly effective, especially vs green.
Rune-Scarred Demon A little expensive for mana, but getting a bonus 6/6 flyer with your Demonic Tutor is pretty sweet.
Bottomless Pit and Oppression - Classic discard strategy from black. This can be incredibly frustrating for your opponents to deal with. Just be sure you don't have anything you're too fond of in your hand before playing them.
Cut / Ribbons - New to the scene, this card can be quite effective. The first sorcery isn't anything special, but the aftermath ability can finish off anyone who's pretty low on life.
Emrakul, the Promised End - Why not add some flavor with a huge Eldrazi? I wouldn't recommend running this if you don't know how your opponent's decks work, but it can easily remove someone from the game if you can take full advantage of it.
Spiteful Visions - A very popular card in Mogis, this can make drawing cards a hassle, especially vs blue. I personally don't run it because I don't like giving my opponents card draw if I can't punish them enough for it. This runs much better in a Nekusar, the Mindrazer deck as it focuses around hurting players by drawing cards.
Fate Unraveler - Another way to make players hate drawing cards. This can be run in tandem with Underworld Dreams, but enchantments aren't as easy to get rid of as creatures (comparatively), so I only run Underworld Dreams in my deck.
Liliana of the Dark Realms - She can be useful if you're low on mana and need to grab some lands. She's only really useful if you get her ult off, but that isn't likely to happen. Still, I have found use for her in my deck before and it isn't a bad consideration.
Palace Siege - Passive life loss for your opponents goes very well with this deck. However, since it is only limited to your upkeep, I am hesitant to run it myself.
Tree of Perdition - A funny way to make someone drop their life total in an instant. If you want more silliness, run this with Triskaidekaphobia.
Rakdos, Lord of Riots - If making this deck more creature-centered, then this is your guy. A popular commander on his own and with good reason.
Crypt Ghast - Great if you need some mana ramp, but I haven't had much trouble playing without it.
Sin Prodder - Since our deck is full of big bombs, this guy adds some extra damage to the mix against people who don't want you to play them. Can you really blame them?
In Garruk's Wake - First off, yes, it's 9 mana. Which is why I don't main board it. But it doesn't affect you and it also does well vs planeswalker decks. It can seriously come in handy.
Exsanguinate - It goes without saying that this has the potential to win the game for you. No doubt that this should cross your mind when building your deck.
Toxic Deluge - Another excellent board wipe. Just be aware that this can easily backfire too.
Curse of the Cabal - Yet another card sure to make you giggle. If you can't pay the mana cost or want to add some politics, suspend it instead.
Ankh of Mishra - Has the same effect as Zo-Zu the Punisher but in artifact form. Running this with him and Polluted Bonds can be very strong indeed.
Smokestack - This is just plain obnoxious to deal with. Forcing everyone to sacrifice their stuff is great, just don't go too crazy and screw yourself over.
