Sorry for necroing a thread but:
Rituals are a good idea.
Usually rituals are performed through several acts such as sacrifice, drinking of somthing and then a chant; just as an example.
Usually to cast a magic on a certain group of people or person which effects them extremely powerfully and for longer period of time.
They can also be used in raising or sommuning a creature such a deamons and the dead.
Single person rituals are usually linked to vengance or to get back at a single person, and require a slightly simpler procedure to complete: light a few candles in a pattern, chant away, pour blood onto an object of power. This can cast a disease or somthing of the sort on a person, or even strike them down. They can appear strange to other people and be hated by everyone because of this ritual performed on them.
Other rituals such as cult extremeism are just used to please a chosen deity, usually involving sex acts of some kind and alot of times abuse of a person or being.
If a ritual is based on one person, I think it usually involves having somthing of theirs, such as a hair or item etc.
Ii;m not entirly sure how this would fit into game worlds, but maybe you could choose the type of ritual you wished to perform from a list or somthing and the person with the best skill involving that type of ritual would lead the ceramony. List could be like "cult worship", "powerful magic", "singlular rituals", "summonging" etc.
The more experianced you are in your chosen ritual field the mrore rituals you can do without dire consequences. If you are not skilled enough in channeling the power you are trying to summon, the magic could backfire and cause horrible side effects. You would also have to search around to find new rituals, it would not just be handed out when you train, you might find it written down on a piece of parchement dropped by a dead fanatic, or a rogue who stole it from another person etc. Large books in the library could hold a few passages and instructions on how to perform a ritual or two.
Maybe you would have to keep the parchment in your mind slot, to remember it, as rituals take time and effort, and step involved etc. its not easy to remember all of those steps when you havnt performed it in a while or you havnt reviewed it. best to be sure and have the spells fresh in your mind, else the consiquences could be horrific if somthing is done wrong.