Background

Of the great Cabal, much is speculated and little is known. The organization is said to be amassing both political influence and otherworldly knowledge, although to what end is unclear. It is said its members are present in every court and council, pulling the strings in secret.

Although its agents are abroad across the realm, the Cabal has its meetings in the ancient ruins of the fallen Empire of the Sphinx. These dune-swallowed towers, caves, and temples make for the perfect clandestine hideout.

Its enigmatic leader, The Deathspeaker, is an immortal sorcerer who has made a pact with Hell. Those who join the Cabal are sworn to service, in this life and the next. He is, in fact, the last surviving member of the Sphinxian people, and the traitorous advisor responsible for that Empire's calamitous fall at the hands of fell powers. He retains an affection for the deadly scarabs that were sacred to his people, which have become a symbol for Cabalist allegiance.

Strategy

Cabal is a control/combo deck with a focus on card draw. This heavy draw is key to several of the deck's strengths:

Especially when combined with Deathspeaker's ability, you should always have a strong play and a way to spend your mana
Maximizes odds of having an answer to whatever your opponent runs, especially with silver bullet for tech cards
Maximizes odds of drawing the deck's best finisher, the Nimbus Djinn, and enables its ability to instakill easily - Djinn is not necessary to close out the game, but it's damn good and fun to boot

Other notes:

Highland Falconer is by far your best opener. Pull out Scarabs to deal with big minions or Bats to clear smaller ones and keep yourself healthy - especially against Necromancer or other token decks, bats are a lifesaver. Also, make sure to always Falconer from the deck, even if you have a valid target in hand (maybe obvious but bears mentioning)
Focus on controlling the board, not damaging your opponent. You are quite happy to trade and bide your time for a sudden, decisive kill with Djinn or, barring that, just using burn spells and big minions like Legion of Gall
Vampires are just a super strong minion to help control the board, top up your life, and then chip at the opponent when their main job is done
Sometimes you'll have spent genesis dorks (0-1 power) you want to die so you can Speaker them. Temple of Moloch and Ring of Morrigan are great for this, and sometimes I'll even Imp my own guys, but I mostly just use them as chump blockers
It's worth keeping a single spellcasting dork alive for Silver Bullet and Pact
Spore spouts are so useful with Imp, Hotwheel, and especially Bane Widow, and seriously reduce the necessity of Summoning Circle - remember to Spores for the recast from Speaker as well!
Smokestacks of Gnaak is nice if you play it next to a Wizard's Den that is already placed
Balor can kill burrowed/submerged minions with the beam, making him a good counter to Root Spiders, Orm Infestors, and such. He's also just a strong body and a good finisher
Here's a fun combo - if you play a Grandmaster Wizard onto the Geistwood, have him cast Pact, and then Speaker him, you draw 12 cards!

Change Considerations

If you struggle to get your dorks killed, I'd see about adding Flayers. I used to run Cauldron Crones for this but I think Flayer is better and more multipurpose
I like Blink as general utility and a way to get Djinn in position, but it is not essential, and could maybe be swapped for more Spheres. The risk of this is that any Spheres drawn after the first tend to be dead in hand, but at least you can feed them to the Djinn
I used to run Mixes, and they weren't exactly bad, but I often found them redundant. If you have trouble getting off the ground they could be good though

Cabal

@Hemlocke