I could stand some more hype.
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We will get along fine since I mainly focus on one thing. Any deal that Vakarlon feels like making. Example:
Mortal: "Oh, great deities, I pray that you grant me good fortune."
Vakarlon: "Can I have your soul in exchange?"
Mortal: "Um, what do you plan to do with it?"
Vakarlon: "Some people collect coins, some people collect stamps, I collect souls."
Mortal: "Okay then..."
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Don't think Demons is wide enough to be considered a Domain to be honest. It seems like something that would come under 'Magic' or - as Cyclone suggested - 'Evil', or 'Occult'
In fact, you could go go for Magic (Pacts) - so the pacts will be binding due to the magical nature of the agreement...and it would also mean that you could grant people some kind of 'Pact' magic which makes them create magically enforceable agreements...
Though mind you, Astarte is already a goddess of Magic
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Eh, I'm stumped. Pacts, deals and contracts are all basically 'enforceable promises' of some kind - whether legally binding or binding for some other reason. Perhaps Promise as a Domain with 'Contracts' or 'Oaths', 'Pacts', 'Vows' and so on as potential portfolios?