So I'm a huge fan of the Dimir flavor and graveyard/exile manipulation. I've been working on this deck since Gatecrash..
You basically get three options for winning: Milling, unblockable Aberration and stealing creatures.
Here's the rundown:
Ashiok: One of the win conditions, stealing creatures is huge against a lot of relevant decks like red and black devotion. It also draws a lot of hate and of course shrinks your opponent's deck.
Breaking // Entering: This is a super important card. To win by milling you generally have to get rid of 45-50 cards. If you consider the deck a second life bar, then this is is just over 3 damage. More importantly, it feeds a ton of cards into the graveyard on turn two after DRS so ramp becomes easier. It's also crucial for buffing the Aberrations. Finally, it leads perfectly into Crypt Incursion.
Consuming Aberration: Just about anything you do buffs this guy, he facilitates milling, protects your planeswalkers and is a major win condition against fast decks when combined with rogue's passage.
Crypt Incursion: Extremely crucial in many matchups, using this after Breaking or Jace can win you precious turns to set up your win conditions. It's also the card you'll pull out against creature-weak decks for the sideboard options.
Devour Flesh: Beats any creature at 2 cmc. It trades in precision what it makes up for in reliability.
Jace, Memory Adept: This guy is a clock. Draw cards or speed up the mill process by a hundred-fold depending on what the situation calls for. Really flexible card.
Nightveil Specter: Card Advantage and mana-fixing on an evasive body. Super flavorful card with the rest of the deck.
Psychic Strike: It's a counter that mills. You already know what to do.
Temple of Deceit There are a couple nice gaps where you don't need mana right away, usually on turn 1 or 4. Scrying can save your life and just makes everything more consistent.
Thoughtseize: The pillar of black control cards. Ruining your opponent's hand buys you time. It can also force a top decking war, which gets increasingly frustrating when you're milling their playsets.
Mutavault: This deck's weakness is weenie aggro, so mutavaults help add chump blockers without screwing up the balance of the deck. Not as important as in other decks, but still a huge help where it's needed.
Sideboard:
It's pretty simple. Swap the Specters with Zombies against green and white. Pull out the Crypt Incursions and an Ashiok for the negates against Esper Control. Add in the Cures and Ratchet Bombs where relevant against weenie aggro. Pithing Needles come in against Esper Control and anybody else using annoying activations.
