We've seen Simic Flash in Standard in the past, but Ikoria: Lair of the Behemoths gives us some sweet new tools to make Dimir Flash the cool new kid in town.
Dimir Flash Decklist (Standard)
4 Brazen Borrower/Petty Theft 2 Ashiok's Erasure 4 Brineborn Cutthroat 2 Neutralize 3 Sea-Dasher Octopus 1 Negate 2 Voracious Greatshark 4 Spectral Sailor 2 Dirge Bat 2 Quench 3 Heartless Act 4 Slitherwisp 4 Cunning Nightbonder 2 Castle Locthwain 2 Temple of Deceit 2 Zagoth Triome 4 Watery Grave 7 Island (335) 6 Swamp (339)
Slitherwisp and Cunning Nightbonder are the cards that draw us to building this archetype in the first place. Other versions of Flash decks like this have seen a smattering of success in Standard at various points in their Standard lifetime, but one of their weaknesses was always that if they fell behind on card parity it could be difficult to catch up from behind. Slitherwisp completely blows this weakness out of the water, letting us play as some hybrid combination of control, aggro, and combo all at the same time. Cunning Nightbonder can enable some of the particularly nasty sequences, turning all of our 2-drop flash creatures into 1-drops.
Sea-Dasher Octopus is a particularly intriguing card from Ikoria. It does a pretty great impression of Curious Obsession when you are able to lead on Spectral Sailor, an opening that led to Mono Blue aggro being a feared deck in Standard in the past. With Cunning Nightbonder in play it is easy to just cast this for a single mana and hold up protection to help your valuable creature to connect, though it is worth noting that Nightbonder is a human creature and thus cannot be mutated itself.
Brineborn Cutthroat and Brazen Borrower continue to be the powerhouses in this archetype that bring our power level high enough to contend. Brineborn Cutthroat is even more potent when we are drawing extra cards and casting them at reduced costs, and the expensive mythic rare continues to be great at providing both disruption and a clock in a deck that seeks to play disruption with a clock.
Voracious Greatshark is an awesome card that we should play as many copies of as we can fit without being greedy on the curve. Some of the easiest games you'll ever win are when your opponent ramps into a threat, you cast Voracious Greatshark, and then they die two turns later. For anyone who's played with Torrential Gearhulk, this card does a strong impression.
After the key synergy creatures, the deck needs to be rounded out with a smattering of removal and counterspells. The decklist posted above is a good example for a starting point, but this is an example of a deck that needs to be highly tuned for a specific metagame. At the time of writing this article, hard counterspells have been pretty good, as a lot of people are trying to play big spells such as the Gyruda combo deck.
Ashiok's Erasure is an interesting card for this deck that wouldn't normally be considered otherwise. However, it is pretty sweet that it has the actual keyword Flash instead of just being an instant, so that it benefits from Slitherwisp and Cunning Nightbonder. It's possible that this is just too cute and we should stick to the 2-mana counterpspells, but Ixalan's Binding was an important part of the Standard metagame when it was legal so we see no reason why this card can't be played in a deck that plays entirely at instant speed. This is even more true for a format that seems to have many decks that revolve around a specific card, such as Fires of Invention.
Dirge Bat is a pretty interesting card to consider in this archetype. It is not particularly efficient up front, which is unfortunately at tension with the core strategy of this deck. However, it does provide the potential for repeated removal and another flexible card that can benefit from the cost reduction of Cunning Nightbonder. In our games playtesting with it, it was much more common to simply cast it as a 4-drop clock and have the potential to mutate onto it later with Sea-Dasher Octopus. It's entirely possible that we will see different builds of this deck be successful, some focusing more on the powerful synergistic interactions and some focusing more on the raw efficiency cards such as Heartless Act and Murderous Rider.
Check out our video piloting Dimir Flash through a few games: