Ritual magic is just magic that takes longer than normal magic. Use the same spell list as normal magic, but treat casting time as minutes instead of turns; if the casting time is already in minutes or longer, it's already a ritual magic spell.
The big question is why anyone would want to do this. Most settings with ritual magic give it one or more of the following benefits:Shared cost with others
: Using the number of ritual celebrants involved, look up the multiplier on the mass combat chart
and divide the total hit point cost by the multiplier; each participant in the ritual loses that amount of hit points, while the leader of the ritual loses twice that. All participants must either know the spell (be high enough level to cast it) or be instructed on their role in the ritual by someone who knows the spell.Sacrifices
: Willing or unwilling victims can be maimed or slain as part of a ritual; hit points of damage are deducted from the total spell cost before figuring the shared cost. GMs might allow similar deductions for other kinds of sacrifice, like body parts from magical beasts or dust from ground-up gems.Tweaking spells
: Ritual spells can have their range, area of effect, or duration tweaked. Doubling the range or duration doubles the hp cost. Doubling the casting time halves the hp cost. All the large-scale magic, like creating a Wall of Thorns around a palace (a la Sleeping Beauty,) is ritual magic. If this rule is in effect, drop the Permanency spell, or any other spell that extends time, range, or area of effect; instead of permanency, tweak the spell to make it last for a very long time, like a century.Pushing the limits
: Magi can cast spells beyond their level limit if they have a written description of the spell (this lets you keep the iconic wizard's library without requiring memorization of ordinary spells.) Since they are normally unable to cast such a spell, there's a penalty: the mage must make a roll against a DC of 10 + spell level to successfully cast the spell, and the hp cost is doubled. Some settings might also allow non-magi to cast/lead in the casting of ritual spells from spell books: treat as a mage casting a spell beyond their level, but double the DC and triple the base hp cost.Eldritch magic
: ritual spells can even be higher than 9th level. For example, a Sleeping Beauty spell would would be a Wall of Thorns (assuming 4th level spell, equivalent to Wall of Fire) combined with Sequester (7th Level) for a total spell level of 11th; cost is 46 hp, further modified by spell tweaking.
The places where ritual magic actually impacts the game are:Prep before the next chapter of an adventure
: If, for example, the party needs to follow an aquatic monster into the depths of a lake, having the mage cast Water Breathing on everyone would leave him depleted. If there's plenty of time, handle it as ritual magic and charge everyone a small amount of hp instead.NPC rituals
: If the NPCs are casting a horrendous spell, you can do a quick calculation to see how long the total spell should take. Set that as the time limit for the PCs to make it through the dungeon in an attempt to stop them. Or, calculate the hp cost and make a roll to see how far the cultists made it through the ritual before the PCs interrupted; this tells you how much hp they have already lost before a combat begins.Scrolls
: Mages can write the instructions for any spell they know, although none would need to do so, except to create an item for sale/trade or to provide a tool for low-level hirelings to carry out their command ("Go to the east wall of the Sealed Tomb and read this Passwall scroll.") Ordinary spell scrolls are treated like ritual magic: anyone can read one in one minute's time and pay the spell cost. The scroll is reusable.
Mages can also enchant a scroll to store some or all of the energy of a spell. Treat this as a 1st level spell, Enchant Scroll, that combines with whatever spell is actually in the scroll to act as a trigger to release the energy. The reader of the scroll must still successfully cast the spell and pay any remaining hp cost (minimum of 1 point.) Enchanted scrolls crumble to dust when used.