http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/arcana/1152

House Dimir values secrecy above all, entrusting only its own agents with valuable codes and information. Whenever you cast a card with cipher, you can exile it, encoding it on a creature you control. Then, whenever the creature slides through and damages a player, you'll get to cast that card again!

House Dimir's knowledge of the undercity can make blocking its attacks nearly impossible. Cards like Deathcult Rogue and Incursion Specialist are experts at navigating directly to your opponent's throat. Set one of them up with a cipher, and you can potentially recast that spell every turn. Send your creature out on a mission to the morgue with Midnight Recovery, returning fallen creatures turn after turn, or perhaps go in for the stealth kill with Shadow Slice. Under such a constant assault, it's only a matter of time until your opponents are overwhelmed.

The "Dimir Dementia" deck benefits greatly from a sparsely populated battlefield. A card like Mutilate from the Magic 2013 core set works wonders, eliminating much of your opponents' planning in just a single spell. Since this deck tends to only have a couple of creatures on the battlefield at a time, you'll be in a better position to recover and take advantage. Also check out Undercity Plague from the Gatecrash set. If you're going to cast the same spell every turn, one that deprives your opponent of life, cards in hand, and permanents is a great way to go!