A good response to the creature-heavy devotion to blue/black decks.
The general idea is to control the board with creature advantage, made possible by various ways of stealing or "co-opting" opponents creatures.
The shining stars are Ashiok, Nightmare Weaver, and her pet, Nightveil Specter, which reaches its full potential when partnered with Chromatic Lantern. (Of course, you would side out the lantern if playing a mirror-match). Dimir Charm is amazing at manipulating their deck to take the exact card(s) you want.
In addition to co-opting your opponent's library, you will also be stealing theirs cards on the battlefield. With Act of Treason, Traitorous Instinct, you will steal and sac, often turning a card's drawback into a double benefit, such as gaining life with Devour Flesh or drawing Altar's Reap. (You can also just sac to mill with Undercity Informer.
Then you will also be permanently stealing their creatures with Simic Manipulator, which can be effective alone but a game-changer with Agoraphobia.
Rounding it out are the brilliant Omenspeaker to get the cards you need and block early, Doom Blade to take care of threats that you cannot steal, and the ever versatile Dimir Charm.
Sideboard: The deck handles creature-heavy decks decently, but has Crypt Incursion to gain some life and buy more time, if needed. The rest of the sideboard is meant to deal with more controlling decks with less creatures. You would naturally side out the Act of Treason, Traitorous Instinct, Simic Manipulator, Agoraphobia, and maybe some Altar's Reap/Devour Flesh for the rest of the sideboard.
The deck is massively controlling and brutal.
