House Dimir

House Dimir, also called The Unseen and The Tenth Guild, is the blue/black guild from the plane and city of Ravnica.
Introduced in Ravnica: City of Guilds, the guild is also featured in Gatecrash and Guilds of Ravnica.

House Dimir is a guild of secrecy, manipulation, and underhanded deals. They are a shadowy organization operating behind the scenes to twist Ravnica to its ends. Previously a conspiracy only the most senior management of Ravnica knew of, the Decamillenial ousted them to the public.
The guild provides knowledge brokering in all its forms, starting from the basics of journalism, public archives, and education; scaling up to blackmail, espionage, and counter-intelligence; with smuggling, burglary, assassination, and other illegal services at the top, with the contacts getting more and more obscure each time.

House Dimir was founded by the ancient vampire Szadek. The vampire was one of the ten paruns who signed the Guildpact and remained in charge of the guild for the subsequent 10,000 years.
One of the crucial clauses of the Guildpact was that none of the signatories (including Szadek) could reveal the existence of the Dimir to Ravnica's people, protecting the guild's anonymity while forcing it to remain hidden. This created a flaw in the Guildpact's structure that would eventually allow it to be broken.

A good Dimir deck controls, destroys, and uses trickery to allow for evasion or, making your opponent’s deck work against them.

• Main races: Faeries, Drakes, Horrors, Specters, Spirits, Vampires, Zombies
• Main classes: Rogues, Wizards (particularly Necromancers), Assassins

Clan Silumgar

Silumgar is one of the five elder dragons of Tarkir. He is a fat, snake-headed creature with a corrosive, venomous breath that hoards valuable items. He is destructive, cruel, and murderous.
Of the five dragonlords on Tarkir, Silumgar had the fewest living followers, but the clan's undead numbers were unrivaled.

Silumgar's underlings emulated their dragonlord in all that they did. They took care to curry favor and deflect blame. Assassination and extortion were commonplace as those in the inner circle jockeyed and killed for a position.
The naga translators were often the first and fastest to gain favor. When Silumgar wanted to lay waste to his enemies, he called on the Deathbringer dragons.
They breathed clouds of poison gas that choked armies and drooled streams of acid that dissolved everything in their path.
Meanwhile, some necromancer dragons would enhance their terrible magic by pulling the latent energy from dead flesh.

The dragonlord himself had grown paranoid about losing everything he had gained to Kolaghan or that the Rakshasa might turn against him as they did with the Sultai.
In these fits, he often killed his subjects. When he discovered that someone had attempted to undermine or manipulate him, his answer often was swift and brutal, rooting out whole networks to punish sedition.
As a result, numerous festivals and rites were designed to distract, soothe, or delight the dragonlord and keep his thoughts away from paranoia.

• Main races: Zombies, Humans, Nagas, Dragons
• Main classes: Assassins, Warriors, Wizards

The Speed Demons

The Speed Demons is a racing team from Duskmourn that competed in the Ghirapur Grand Prix. They are a group of possessed racers and their tormentors who drive specter-haunted vehicles, led by the eponymous Speed Demon that allows their excursions from their homeworld.

They are the advancing edge of Valgavoth's "mansionization" process across the Multiverse. The Speed Demon itself races among the possessed and the damned as a demonic minder, the tormentor of the team captain, Winter.
Winter is a survivor of Duskmourn who has been granted temporary leave by Valgavoth, the archdemon of the plane, to try and win the Aetherspark.
If Winter (or a racer on his crew) wins, he might be granted his freedom. If he fails, however, the Speed Demon will ensure Winter is dragged back to Duskmourn to face Valgavoth's wrath.

Winter's prospects look stark, save for one ace in the hole: Loot, a young beast with an uncanny ability to navigate the Omenpaths.
Winter and the Speed Demon held Loot captive and planned to take advantage of his abilities in the race.

• Main races: Humans, Demons, Spirits, Beasts, Toys, Scarecrows
• Main classes: Pilots, Warlocks

The College of Theorix

Theorix is the School of Esoteric Mathematics at Hexhaven University on the Echoverse version of Arcavios. It is aligned and the counterpart of Quandrix. The founder of the school is TBA.

Theorix is the mirror of Quandrix, but is instead of .
While the description of this College is a little vague, with esoteric mathematics also feeling like a somewhat apt description for Quandrix, it's safe to say that with the color shift from green to black, we can expect more spells that attack the power and toughness of creatures, rather than adding counters to them.
Not much else is known yet, due to the set and story not being released yet.