The Ancients sometimes saw little difference between the ways clerics and magic-users cast spells. All things came from the Gods after all, including magic, so both classes would use similar methods at times. This fact is reflected in game terms by not only the custom spell lists available in Divinities and Cults: Volumes I & II, but also in the boons.
Below are a number of other boons available for spellcasters in your game, so that they can increase the usual chances of a spell’s success or even attempt greater elaborations (q.v.) Inspired by the historic Greek Magical Papyri, and all its many cultural influences (in addition to Greek), here is part 1 for your gaming enjoyment.
Appeasement (+1, +2, or +3): Provide food, drink, and any other gifts that would appeal to the being that is summoned. The more appeasement provided and the greater its relevancy, the greater the bonus. This boon only works for spells that would summon beings, of course.
Scribing (+3 or +5): Carve special characters on a fitting object to help in the spell. The process usually takes at least 1d20 minutes to finish the carving, in addition to the time needed to either fabricate or acquire the object (per Referee). The particular item can then no longer be used for this boon, if and when the spell succeeds. The higher bonus is given if the item is particularly valuable.
Smearing (+2): Cover the target’s body with a substance or substances that match the spell’s intended effect and/or the cultural tradition of the caster. It takes at least 4d6 minutes to apply and also 1d4 hours to get the unpleasant smell off the target, once the spell is finished. The target suffers a -4 penalty to all Charisma checks, as well as Saves vs. Magic during that time.