Earthbound:

Demon: the Fallen interact with the rest of the World of Darkness in a unique sort of way. For starters, they are the only setting that not only encourage cross-sphere, their cosmology actively promotes it. Why? Because they are literally everyone's enemy.

I'm not going to write a big thing on how Demons interact with the other supernaturals, because most of your characters won't have been free of Hell long enough to have much experience with them, and while you have existed longer than them, Hell didn't have a great parole mechanic. Therefore, while Vampires, Werewolves, Mages, Imbued, Wraiths, etc, all exist within a D:tF game, most won't know much about them. We're not going to be creating a complicated Lore system to represent your knowledge of these other things: we're just going to roll that into Occult. Knowing how Werewolves or vampires or whatever work is covered by Occult. Knowing how they interact with D:tF comes from experience, beware.

But while D:tF might be enemies with most other supernaturals for more esoteric reasons, their real enemy are the Earthbound.

Earthbound is a broad category of D:tF that are trapped on Earth (as opposed to Hell) inside inanimate objects called Reliquaries. Earthbound come in all shapes, sizes, ages and power levels, but when we're talking about the biggies, here's a list of the ones we're talking about:
Asmodeus, a Neberu whose visions revealed that humanity would rise above the Elohim and hated them for it.
Abaddon, a Rabisu who conspired with Asmodeus to supplant humanity with the nephilim.
Azrael, a Halaku who assisted in the nephilim's perfection, and collected the souls after their destruction.
Belial, a Lammasu who was once Lucifer's greatest supporter and most trusted lieutenant, and is now, as the "Great Beast", his fiercest enemy who exists purely for his former hero's destruction.
Dagon, an Annunaki who also advised and was held close by Lucifer. He now desires to transform all of humanity into soulless, mindless clockwork beings, much in view of the Technocracy.

Any character with a Legacy of 3 or more remembers the great Arch Dukes of Hell. These were Lucifer's most trusted supporters, and they vanished from Hell sometime in the beginning of recorded history (that is to say, somewhere between 5 and 7 thousand years ago). These are the big ones.

Lesser but still well-known Earthbound include:
Avitu, a former Angel of Knowledge and Understanding, now Demon of Ignorance
Baal, a Rabisu who claims to be an Archduke, even though he was a lowly fell knight. He has accumulated an impressive amount of power through his claims.
Enshagkushanna, a vassal of Belial.
Kupala a demon that dwells within the soil of Transylvania.
Manishtusu, the Lord of Murder, who controls and manipulates the street gangs of Los Angeles.
Nikanuuranu is a Neberu that has formed a pact with the Inconnu's inner circle, the Council of Twelve, to conceal their base from the outside world.

Anyone with a Legacy of 4 or more can remember these names and their significance.

All it takes for a Demon to become an Earthbound is to be trapped on earth in a non-human shell. This is easier than it sounds. Earthbound are also unique in a few ways.

1) They gather Faith much slower than their fleshy counterparts. It may take them a week of dedicated worship to gather a single mote of Faith. Of course, if their Cults are big enough, they can still gather several Faith a day via worship.

2) Because they aren't constrained to mortal flesh, they are able to have a Faith score higher than most humans would allow. The biggest 5 Earthbound are rumored to have Faith scores over 30. Also, their temporary Faith can exceed their permanent Faith.

3) They get a unique set of Lores that are difficult to acquire (so young ones rarely have them), but can be devastatingly powerful.

4) They all have Torment scores of 10. This is the big thing. Every Demon in Hell was driven mad by the experience. Even the most compassionate Fallen before being locked in Hell is universally howling for the blood of Creation now. Hell isn't nice. And, without a human host to dilute that Torment to something reasonable, without a mortal set of eyes to view the world... the Earthbound are all still insane and wanting to either end, enslave or in some other way control Creation.

5) Earthbounds that have Faith scores beyond 15 are no longer bound by the same laws of Pacts. They have their own rules for Pacts, and they can Pact with things that might not normally be vulnerable to such bargains. (Dark Thaumaturgy is rumored to come from a very specific Earthbound...)

6) Just as Humanity hasn't diluted their Torment, nor has it diluted their memory. While they might not be able to access it in it's entirety at any give them, all Earthbound have shockingly clear memories (as represented by Legacy 5).

Earthbound are very dangerous. They represent true threats to Creation, the Fallen, Lucifer himself, everything. That said.... they're vulnerable to all the same limitations of other Fallen. Should someone ever complete one's True Name (and figure out the binding ritual before its servants obliterated the creature completely), you'd have one boss slave. Like... seriously.