Not so much for the contractual part, though.

That, I believe, is covered by devils.

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.

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.

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.....

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'.

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?