So I thought I'd start off this modern occult setting by giving you a broad overview of the setting. It cribs a bunch of ideas from WoD Mage (mostly Awakening), but I feel like I made it my own and is cool/metaphysical enough to stand by itself.
The basic idea is that the universe is sentient, kind of. It's more like the fabric of the universe is all powerful and somewhat knowing, and the term for this power is "Om" (after the 'noise of the universe' of a lot of Eastern religions).
The Om is the entire universe, but it can't really influence anything; it's just there. So the Om created the Councils and the Fundaments.
The Fundaments are inherently 'human' realms, related to the four classical elements. The Fundaments are really the 'make up' of Om, and these Fundaments created humanity. These are the most basic building blocks of reality, with Om representing a kind of 'quintessence'.
The Councils control the more fringe, but still important, aspects of reality. There are three Councils, each ruled by an Arcana. The Arcana are indeed based off the Major Tarot, but the actual Arcana existed before their labels did. Each Council (named The Court, The Forum and The Judge) has three Houses underneath them, each of which is ruled by two more Arcana. Thus, each Council has seven Arcana.
Each House also controls a Realm Beyond, a metaphysical realm where their chosen aspect derives from. Those will come in later when I describe each Council in turn.
So for millions of years, this worked. Then humans gained sentience, and things went to pot really quickly. The Fundaments grew much more powerful than the Arcana, and they became capricious and jealous. They basically fractured off and started an all-out war versus Om and its Fundaments. This war was staged both on the cosmic arena and on Earth. The Arcana released all manner of terrible creatures, fell magic and disasters across the universe.
The Om was losing, so to remedy this, it invested some of its power in both chosen humans and rebellious creatures of the Arcana. These champions became known as Instruments, but there was inevtiable infighting both within the factions and between the humans and the otherworlders (their factions called the Order and the Cabal, respectively)
The Instruments needed humanity for one reason or another, so though they often fought during lulls in Arcana attacks, they would still band together in unlikely teams to stave off these cosmic threats to humanity. What's worse is these attacks came in waves, and many Instrument leaders fear the largest attack ever is just around the corner.
So that's where the game starts. You are a human/monster/mage/whatever, who get together in ragtag groups to fight the Arcana for the good of the Earth and the good of the universe. Ideally, you start off with only knowing that you are fighting creatures from other worlds; as your characters reach higher levels, you learn more about the 'truth' of Om and the Arcana.