Control through discard.

Discard:

Duress/Thoughtseize - I can't run more than 4 thoughtseize, and seeing your opponents hand in the beginning is very important. You'll also need to know if you can get a discard off later in the game, so knowing if they have counters, or are just holding land to sacrifice to your discards. Most likely getting to pick a card they discard is also useful. Knowledge is power, france is bacon.

Mind Rot - This card has always seemed kind of expensive to me, but long gone are the days of BB opponent discards 2 cards. This is a staple card, the second highest discard effect. While it doesn't feel efficient, it's the best we can do for discarding 2 cards.

Read the Bones - Black's best draw card. Almost as good as Sign in Blood, but with 1 colorless for scry 2. My best source of clumping fixing.

Skull Rend - Strictly speaking, this card is not worth it, but it's only off by 1 colorless mana. R for a shock + 2B for a mind rot. Obviously this would see more use in EDH, but for now it's just a late game discard, with some extra damage. It's not a great card, which is why I only run 2, but options on discard are limited.

Rakdos's Return - Awful grammar aside, this is going to be the main source of both damage and discard. Mind Rot is more efficient at 2 cards, but I'd rather cast this with x=3 than a Skull Rend. Early game it's a less efficient Mind Rot, mid game is when it's most powerful, and late game it's a finisher. The reason it's most powerful mid game is because that's about when things like Jace and Opportunity will start seeing play, at which point you're more likely to get full effect from Rakdos's Return

Slaughter Games - Great card. Can't be countered, and can get up to 4 cards. If you pick something you know your opponent has a lot of (Dissolve, Elspeth, etc.) you have the most efficient discard in the game. This is also why I don't run Pithing needle. Granted, it needs to happen on turn 4 to be most effective, it is still incredibly efficient and really useful for removing your opponent's answers. Bonus is that even if the named card isn't in your opponent's hand or library, you still get to search for them. Hello counting what they have for the next one. Second bonus in that the cards are exiled. The only drawback is that it doesn't get cards on the battlefield. I only run 2, but running 3 or even 4 is a consideration.

Creatures:

Deathrite Shaman - This card is incredibly good in any deck that can run it. It synergizes fairly well with discard, since there's a decent chance you'll get land in their graveyard. It can ping down most decks and with two Shaman's out, it can even gain you some life. This gives the deck some mana ramp and an alternative source of damage. Also helpful for getting problem cards out of graveyards.

Sire Of Insanity - Only an ok card, and has some synergy with the rest of the deck. It's 6 cmc for permanent discard and a 6/4 threat, with the drawback that you have to discard as well. Obviously late game. Works best when one or more Shrieking Afflictions are on the field. Unfortunately with the prevalence of enchantment removal, Shrieking Affliction never sticks around for long.

Blood Scrivener - One of the best cards in the deck. It synergizes incredibly well with Sire of Insanity, somewhat negating the Sire's drawback. Better than the old Sign in Blood, but is fairly easily removed. In a pinch, having one on the field and casting a Mind Rot on yourself can get you two cards on the next draw. Not the most efficient combo, but it's there. While this is a good card on its own, I'm using it as a patch on Black's lack of reliable draw and mitigating Sire of Insanity.

Enchantment(s):

Shrieking Affliction - A worse Megrim. Cheap, though, and enough of a threat to be counter bait. Due to enchantment hate in Theros, I'm only running 3. It has the most synergy with Sire of Insanity, who guarantees the 3 per instance loss of life. Obvious synergy with the rest of the discard, too, but much less reliable in that regard.

Other Considerations:

Dreadbore - I decided to main deck 2 of these due to Planeswalker threats in all of the top decks. Good otherwise for spot creature removal.

Land base - A little low. I like 20 lands, but avoiding not having enough mana is a good thing. I currently have 22 in, but could potentially bring that up to 24 by tweaking a couple of my 4 of's. Diversity for colors is fine, although I may have one too many mountains.

Sideboard - Main deck lacks the ability to deal with creatures after they hit the field. I feel like that is its biggest weakness, because things like Young Pyromancer, Master of Waves , and so on could become a problem. Volcanic Geyser is in there to replace Dreadbore in those rare cases my opponent has neither creatures nor planeswalkers that I care about (looking at you Liliana). Duress sideboards in against the heaviest Control decks.