Dimir Flash Nightmare

Ah, Blue-Black, my favorite color combination in Magic.

While certainly not as powerful as the mostly-green deck I've put together with stuff like Wicked Wolves and Questing Beasts, I burned just a couple of wildcards to put together an U/B flash deck that, while not always effective (certainly still working out some kinks) is pretty fun.

The premise of the deck is that everything in it other than lands is either an instant or has flash. As a result, you only really play anything on the opponent's turn.

Some core cards:

Brineborn Cutthroat - these guys are cheap, have flash, and any time you cast a spell on an opponent's turn, they get a +1/+1 counter. Given that you're going to be doing that with just about every spell, they're going to pump up almost as fast as an Ajani's Pridemate.

Slitherwisp - this is the one I burned two rare wildcards on. While the deck probably wants a full 4 (they're cheap and the effect both stacks and is very good) I always try to keep a reserve of wildcards just in case - psychologically, so I'll know that if I discover some insanely powerful new card I can always get it. This is an Elemental Nightmare (fitting with Ikoria's color-themed creature types) for UBB with a 3/2 body and flash. Whenever you cast something with flash, you get to draw a card and your opponent loses 1 life. I think the card is the real advantage here, though the little life drain is certainly not bad. Granted, the more instants you toss in (I just moved in a few Eliminates, Tyrannical Scorns, and Drown in the Lochs, which we'll see about) the less valuable this card is, but it's still pretty nice to have. It is, however, the big rare card of the deck.

Cunning Nightbonder is one of the hybrid-mana human bonder cycle. This one's a 2/2 for U/B U/B, and reduces the cost of spells you have with flash as well as preventing them from being countered.

I've found Pouncing Shoreshark to be quite useful - I'm used to using it in pure mutate decks, where I'm really just building up a giant monstrosity, and in those cases I get the use the effect over and over, but a single bounce and bumping one of the Brineborn Cutthroats permanently up by +2/+2 is not terrible.

A lot of the deck, then, is just about all the flash nonsense I can toss in.

EDIT:

So it's funny, because the deck doesn't actually have a ton of U/B cards from the Guilds of Ravnica block, but I realized that the whole vibe of the deck is, in fact, very House Dimir. You almost never cast spells on your own turn, and that means that the other player never knows what the board looks like. They can attack, and by the time you're ready to declare blockers, there might be multiple new creatures on the board and your 2/1 merfolk pirate is all of a sudden a 5/4 shark beast.

The deck, or at least decks like it, isn't unheard of in the metagame, so against experienced players you're not going to utterly shock them, but one element of the deck is that you really heighten the unknowns for the other player. When playing against a green deck in which they've suspiciously left some lands untapped, you tend to know that there could be a combat trick of some sort lurking there. But generally speaking, you can usually trust that if there aren't any creatures on the battlefield, you're safe to attack.

The lore of House Dimir is all about how secretive and subtle they work - they went for 10,000 years without anyone knowing that there even was a tenth guild in Ravnica. So even if the showcase cards here are from Ikoria and, I believe originally Ixalan (could be wrong,) Szadek would be proud.