Sultai Delver/Midrange. I'm trying to complete the BGx Trinity, so look at this as being essentially "Blue Jund".
I elected to go the Delver of Secrets
route with it.
Gameplan: Depends on the opponent. Sometimes you want to play proactive, and sometimes you want to play reactive. This deck does both fairly well, but the hard part is determining which route to take and when to switch.
Against combo decks, you want to play a creature as early as possible to put pressure on your opponent and force them into a "Just go for it and hope you don't have a counter" situation.
Against control decks, you play your threats early and save cards like Stubborn Denial and Simic Charm for protection against removal. Liliana of the Veil is also really good here.
Against aggro decks, you have to kill and counter aggressively and hope for good topdecks. And then you have to know when to turn the corner and start slamming them with your 'Goyfs.
Against Midrange decks, you just play midrange. This is our easiest matchup.
Now for the sideboard.
Dispel: Counters opposing counters and removal. Also good against Burn. Notably hits Ad Nauseam.
Collective Brutality: Great against Burn, small creature aggro decks, and combo decks that win with instants or sorceries. Tends to be good wherever Simic Charm isn't.
Bontu's Last Reckoning: Board wipes shouldn't need explanations. It's an easy way to flip an entire game in your favor.
Disdainful Stroke: Offers coverage that combines with Abrupt Decay. Very relevant against a lot of decks. If their win condition is four mana or more, side this in. It's that simple.
Dismember: Extra removal spell that hits a lot of things that Fatal Push doesn't. Competes for this slot with Murderous Cut.
Golgari Charm: Hits Leyline of the Void, Blood Moon, and any other problematic enchantment. Wipes the board of those annoying Lingering Souls tokens, Eldrazi Scions, and Affinity's creatures. Serves as protection from removal and can save an entire battlefield from a board wipe.
Nihil Spellbomb: Grave hate. Replaces itself and adds Artifact to the graveyard to help make up for the loss of your opponent's graveyard for Tarmogoyf.
Ceremonious Rejection: Counters most of what's in Tron, Eldrazi, and Affinity. Against those decks, it's almost a one-mana Counterspell
Surgical Extraction: The combo-killer. Use in conjunction with Thoughtseize, Thought Scour, or any counter that hits the opponent's win condition. Note that nonbasic lands can be targeted.
Thrun, the Last Troll
: Great against anyone that likes to try to prevent you from having creatures. Very hard to get rid of him.
I hope you found my description helpful. If you have any questions or concerns, don't hesitate to comment. Upvote if you think Sultai has a place in Modern.