So I get demons are living, mostly sentient beings born of black mana (not sure how other colors work, like red-white-black Mardu demons or black-green rakshasa, or Rakdos' ilk) but my main concern is the pacts and deals. What do they have to offer, and what do they ultimately want? Your soul? For what purpose? What if your soul is predestined for other things, like Theros' underworld, or in Liliana's case, off-world/promised to another demon?

I'm guessing that some demons gain power through destiny denial magic. Like, they offer you short term (for them) power that will most likely screw you over in the long run and whatever destiny held for you (or your soul if you will) is transferred to them, making them stronger.

I don't think demons really hold sway with 'Walkers the way they do with normal people. That is why Liliana could get away with selling her soul so many times. I'm guessing that the stronger demons that she contracted had help via Bolas in writing stronger magical bonds to tie her down.

Walkers just have more soul. They also feel much more deeply than commonfolk.

Most magic flavour involving demons shows that they use souls as batteries, which fits well into 's ruthless methodology.

Not so much for the contractual part, though.

That, I believe, is covered by devils.

Nope, they're demonic goblins as far as the setting is concerned.

And is the opposite of contractual

Law and order magic is pretty . It's sad that this is never exploited in demons, even if it is the Orzhov's modus operandi

My namesake struck some blood-pacts with the oni, so those seem to be in play on Kamigawa. Generally, being that demons are avatars of greed and ambition, I would suggest that they would demand of a patron whatever it (the demon) most desires at the moment.

Define "bloodpacts". Well what if the person making the pact desires wealth, and his demon patron desires wealth? What do Liliana's demons want with her soul? If it had just been Griselbrand alone for example, and Liliana died, what would Griselbrand truly gain?

Wait a minute... If enchantments are a sign of divinity, divine origins, or divine magic/blessings/curses on Theros, why is there Abhorrent Overlord and Eater of Hope 2 Creature - Demons, but also Herald of Torment and Master of the Feast 2 Enchantment Creature - Demons? What's the difference?

Their origin.
In the same way there's baleful eidolon representing an escaped soul, there's nyxborn eidolon showing a created soul. The enchantment demons are just created demons, while the other two probably started out as actual people.

Pretty much that. All enchantment creatures on Theros are born from mankind's dreams and unconscious on the realm of Nyx.

Oh. So the enchantment demons are created by A. A god(s) or B. Either fearful or depraved mortals? What gods would make demons? I get creatures like Forgeborn Oreads are obviously the work of Purphoros, and a few other specific examples, but who for demons? Erebos? Athreos? Phenax? Mogis? Pharika? I have to believe the former two are unlikely to do so, considering their non-divine origins.....

Phenax and Mogis are probably the culprits.

Even Erebos is not above making enchantment zombies, so he'd probaby make demons as a sign of mockery.

Master of the Feast feels like Phenax's handiwork, and Herald of Torment is Mogis'.

Of course I have uses for this information, but, so a Demonic Pact with a Nyxborn Demon would qualify as such a pact, but such a demonic pact would likely have divine circumstances associated with whichever god gave the demon life?

They could just easily as easily have spawned from dreams or the subconscious, however. Remember, not all nyxborn are necessarily created by the gods.

As the gods claim Nyx as their domain, and create a plethora of nyxborn themselves, what ae th gods' overall opinion of spontaneous nyxborn that they did not create?

Pretty much the same as you would feel about a distant cousin.