I always allow my PCs access to the same strange rituals the NPCs use, if they meet the requirements. Sometimes those requirements are swearing undying fealty to Juiblex and letting an ooze devour their insides to puppet their Hollow shell of a body.Fine, except if the PCs just saw someone use this previously-unknown ritual then that someone has a spellbook somewhere and that ritual's gonna be in it.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.