A Dimir Control build I brewed up for Theros: Beyond Death Arena Standard. It utilizes blue counterspells and black spot removal to control the game until you can safely drop your win conditions, Liliana, Dreadhorde General and Ashiok, Nightmare Muse.

Planeswalker

• Ashiok, Nightmare Muse: The of the best new cards for Dimir colors. His +1 creates a fairly beefy blocker, his -3 can act as removal, and double as hand control. Lastly, unfortunately his ultimate is not as game ending as that of Liliana, Dreadhorde General, however it definitely can bring long matches to a close.
• Liliana, Dreadhorde General: Follows a similar template as Ashiok, Nightmare Muse, among other 5 CMC planeswalkers, for a slightly upgraded kit at 6 CMC. Her +1 helps you “amass your dreadhorde”, along with clutch -4 removal of up to 2 enemy creatures. However, the opponent gets to choose which creatures to sack, and it also wipes two of your own. This is compensated for with her passive, which draws a card for every creature you control that dies. Lastly, her ultimate essentially will end any game on the spot by resetting your opponent’s lands.
• Narset, Parter of Veils: I really love this card for multiple reasons. At 3 CMC, she can dig through your deck for a useful noncreature (this deck runs barely any creatures anyway) up to two times before leaving enough loyalty to stick. Her passive is surprisingly powerful against spells such as Hydroid Krasis, and keeping your opponent’s resources limited. You’d be surprised how many people try to Opt while she’s on the board. Her passive is usually not worth the draw to keep her on the board for 2 loyalty, so don’t be afraid to sack her for the draw though.

• Ashiok, Dream Render: A common sideboard option to deal with pesky graveyard dependent strategies. Most notably anything with escape, or Cauldron Familiar.

Creature

• God-Eternal Kefnet: I honestly love this card too much not to include him, but overall really great utility, and extremely tricky to get rid of. Can also double as a finisher just by repeatedly smacking face over time.
• Murderous Rider: The only creature I really thought was worth running in this deck. One of the best planeswalker removal options for black, while also hitting creatures. 2 life is an easy price to pay when the main body has lifelink to compensate. Note that you can bounce your own Murderous Rider with Tyrant's Scorn to reuse him if need be.

Instant

• Chemister’s Insight: Draw two at instant speed attached to jump-start, meaning you can use it twice.
• Erebos's Intervention: In addition to being capable of countering graveyard dependent strategies, it’s amazing spot removal with life gain. It’s flexible for different situations, not just because of the choose, but due to the fact you can choose how much mana you want to invest into it thanks to the X effect.
• Sinister Sabotage: Arguably the best 3 CMC counterspell right now. An average “counter target spell” attached to surveil which adds an additional layer of consistency to the deck.
• Thassa's Intervention: I honestly am not sure whether or not I want to replace this card. When comparing them at side by side, Chemister's Insight wins in both mana cost and it’s jump-start. However, this card’s added layer of flexibility with the X makes it slightly more appealing. It can double as a counterspell, and for a bit more mana invested into it, it can dig deeper for better cards. Overall, a tentative addition.
• Tyrant's Scorn: An amazing cheap spot removal, which is a powerhouse against decks which run lots of 3 CMC or lower creatures, namely Mono-Black Devotion and any aggro deck. Even if there’s nothing with 3 CMC or lower to target, it can still act as clutch bounce when you’re in a pinch.

• Pharika's Libation: The main reason for running this card is to target enchantments and creatures with indestructible. Can also deal with hexproof and protection.
• Mystical Dispute: Super clutch in blue counterspell battles, which also not being that bad against any deck that runs blue spells.
• Negate: Again, can win blue counterspell battles, and be great against any deck that runs an abundance of non-creatures.
• Noxious Grasp: Cheap spot removal for green and white decks.

Sorcery

• Enter the God-Eternals: This card does an awful lot of things all at once— removal, life gain, mill, and creates a 4/4 blocker. Honestly maybe I’m just biased, but I find it hard to justify cutting so much utility for anything else.
• Ritual of Soot: With the new rise of Mono-Black Devotion, Cry of the Carnarium hit the sideboard for the slightly pricier but more useful black sweeper. I’m still waiting for the day Dimir colors get a Kaya’s Wrath, and can sweep bigger boards.
• Thought Erasure: There’s not really a good justification not to run this card. A Dimir staple, which can reveal your opponent’s resources, force them to discard their stupid Nissa, Who Shakes the World, while also being attached to a surveil.

• Cry of the Carnarium: Despite being overshadowed by Ritual of Soot, it’s still amazing against Cauldron Familiar, or just aggro decks in general.

Enchantment

• Ashiok's Erasure: Great sideboard option to deal with spells that can’t be countered, most notably Shifting Ceratops.

Land

• Castle Locthwain: A card drawing land which can enter untapped.
• Castle Vantress: A land with scry which can enter untapped.
• Temple of Deceit: Finally Dimir has gotten its scry land! Can help smooth out your draws, but will always enter tapped unfortunately.
• Watery Grave: Dimir’s dual colored land which can enter untapped under any circumstances, even on turn 1.