The working draft of the d20
(now 3rd Era
) Freeport Companion
included rules for incantations, largely based on the Incantation
rules from Unearthed Arcana
. The primary changes were that all Freeport incantations used the Knowledge (forbidden) skill, and most inflicted Insanity Points in addition to the usual backlash options. Sadly, after the proofreading phase that I helped with and before the book saw print, that section was cut from the book.
If the Ronins give permission, I might be able to share the deleted text. I also have a version I've edited for use in my own Freeport campaign, adding d20 conversions of the four sample rituals from the True20 Freeport Companion
--one of which is Conjure Demon.