With the addition of War of the Spark, I believe that Grixis Control can be one of the strongest decks in the format. It has answers to nearly everything with the exception of enchantments, but I don't think that will be enough to stop the archetype. I will be looking to climb to Mythic next season playing this deck almost exclusively.

Our main objective with this deck is to be able answer what the opponent is doing until we are able to drop our threats such as Nicol Bolas, Dragon-God and Liliana, Dreadhorde General.

The new Nicol Bolas is extremely powerful and can potentially use another planeswalker's loyalty ability, such as a flipped Nicol Bolas, the Ravager // Nicol Bolas, the Arisen to +2 for faster ultimate use which typically wins the game. Most of our cards get a ton of value such as Augur of Bolas which puts a blocker on the board for aggro decks and allows us to find answers. Tyrant's Scorn lets us deal with any 1-3 drop our opponent plays, slow down a stronger creature by bouncing them to the oppenent's hand, or bounce our own creatures back to save them or for an additional ETB trigger.

Widespread Brutality can clear a good amount of creatures from Mono Red and Mono Blue and some from Mono White. It can clear nearly any board if played after an Enter the God-Eternals. I also really love the 1 copy of The Elderspell. The second match I played with this deck, the opponent managed to get 6 Planeswalkers on the field in one turn and I killed all of them because of The Elderspell.

Sideboard explanations:

Duress - Great against control matchups and planeswalker heavy decks. Also good against Mono Red aggro as a turn 1 play to remove a damage spell.

Negate - Pretty much the same as duress. Great against control.

Moment of Craving - Particularly good against Mono Red aggro for the lifegain, but a good play against any deck that is looking to fill up the board with cheap creatures.

The Elderspell - Like I said above I love this card. Being able to wipe out all of the opponent's Planeswalkers out is extremely powerful, with the added benefit of being able to add loyalty counters. You could potentially play Nicol Bolas, Dragon-God then The Elderspell to boost Bolas up to 8 loyalty and win the game.

Cry of the Carnarium - Another card for the aggressive matchups. Particularly good against Mardu/Rakdos Aristocrats and Izzet Phoenix due to the exile effect.

Thief of Sanity - A midgame threat that can swing the game in your favor if left unanswered. Having this in the sideboard gives you a lot of flexibility and might catch the opponent off guard.

Widespread Brutality - More aggro hate.

Maybeboard:

I threw these cards in there as a potential alternative to Tyrant's Scorn in case you prefer them. I will need to test the deck more to figure out a definitive answer, but for now I will leave these as maybes.