Ritual Magic encompasses a type of magic that can have far-reaching effects and consequences.
Ritual spells are those that require numerous threads and a great deal of time, preparation and research to perform. They can be performed in concert with other magicians to improve the results of the ritual. Ritual spells are usually found in spell lists at above eighth circle.
As of Third Edition, spells of thirteenth to fifteenth circle are all ritual spells. The Third Edition Player's Companion offers examples of these high circle spells. The description of rituals in the guide includes design notes to aid players and gamemasters, and indicates that such spells are part of great legends - implying they are meant to be used as storytelling elements.
A great deal of research is required to create a ritual spell. When designed, they are meant to have an extremely powerful, but very narrow range of effects. They are all Named spells and have a True pattern - for example the Ritual of Thorns.