The Pact System in Elric is quite neat. Flavor wise I don't think it's too far from Sacking Pow for Divine magic. You sacrifice a variable amount of Pow to your otherworldly patron. That portion of your soul is now theirs, you retain it for the purposes of attribute tests or skills. However it now longer generates you MP. In exchange for a bit of your soul you get the following....
A Pact Skill starting at your Pow+Cha, a Number of Gifts and Hindrances depending on your pact score. Gifts could be increased Attributes, Spells and special abilities. Hindrances could be like Geases or detrimental physical changes. For instance the Kraken would give Water Breathing and spells that are water or fear based. While requiring his initiate to drown a sacrifice to him weekly.
Your Pact skill is used for spellcasting gifted to you via your patron. Some Abilities may also use the skill. If Magmo grants you the ability to breath fire in his name, you may likely be using the pact skill.
As it gets higher you get more gifts and hindrances. When the Pact skill reaches 100% your GM will have a decision to make. A Demon Lord will likely either have you dragged screaming to Hell. Or possibly put at the head of some army of the damned. Less questionable Patrons may let the character be for the most part, although they are quite likely to be sent quests for the good more often than not. If you like the characters can just simply disappear, called to fulfill their debts.
As you might tell, I'm somewhat biased towards this Ruleset.