"Calling something ‘ritual’ is a sort of a joke amongst archaeologists" (1) It's easy to understand why. Without the context of how finds were used, who made them, or what daily life was like, it's easy to say that anything out of the ordinary was that way because of ritual use. If our ancestors spent time to carve detailed human figures on the ivory handle of a flint knife then it must have been to honour the Gods, right? (2)
Perhaps not everything out of the ordinary held ritual significance, but we definitely have the feeling that our pre-historic ancestors did an awful lot of odd things which would make more sense if their lives were steeped in mysticism and ritual magic. Classic examples might be Stonehenge, the pyramids of Egypt and Central America, or the Moai stone heads of the Rapa Nui people of Easter Island.
In RPGs that use Vancian magic systems (3) there is a sense that spells are cast by consuming their pattern which was memorised by the spellcaster at the time of preparation. For Clerics and Magic-Users the system is pretty similar, although each has requirements specific to their own form of magic, which is also expressed in the form of different spell lists. Clerics pay homage to Divine or Immortal beings and receive spell-casting abilities, whilst Magic-Users study the hidden machinery of the Universe to unlock arcane secrets which they store in their spell books.
This "every day" magic is great for roving adventurers, who may be sleeping in taverns, camping out under the stars, or resting quietly in dark corners of dusty crypts. However, it doesn't explain some of the more long-lived or powerful dweomers which are often portrayed in campaign settings. For example in the E of BECMI Magic-Users, Clerics and Elves could create new spells or magic items through magical research, the cost of which was time, money, and rare materials. Boiling down this to the essentials we can see that there is a sense in which greater magical feats may be achieved through "sacrifice" of physical (money and materials) and metaphysical (time) assets.
One thing about this system frustrates me, it's entirely a bookkeeping exercise. I would much prefer ritual magic to require creativity on the part of the players, as well as provide hooks for potential adventures to obtain rare materials. I used to have great fun "LARPing" at the Gathering (4) and similar events. In their Ritual Magic system players had to get a group of a dozen or more contributers and put on a 10 to 20 minute show as well as potentially sacrificing magical items or regular spell casting power to provide sufficient fuel to power the desired magical effects. Conforming to the in-setting magical rules as well as being entertaining to watch provided bonuses. A judge would score the ritual using a hidden set of criteria, and the ritualist would learn later what the effect of the ritual was. The system proposed here takes elements from this to give a guide to Referees introducing ritual magic into their OSR-style games.
Sympathy, Theatre, and Sacrifice
'SYMPATHY' is how well the theatre of the ritual and the sacrifice used to power it conforms to the in game magical rules of the setting, up to a limit of the ritualist's level. For example if the crystal core of an Earth Elemental is used to try and create a flying sailboat, and the ritualist's player spends the whole time explaining how cool similar creations are in their favourite anime, forgetting that they are playing in a grim, dark-ages style setting then this score would be particularly low. Conversely, if they had bottled an Air Elemental to be housed within the ship's tiller, and described how they were imploring the divine beings that control sky, storms and trickery to look favourably on their working, they may well score well here.
'THEATRE' is a score of how entertained the Referee and other players were with the ritual performance, up to a limit of the ritualist's level. Rituals should be marked down here if they take too much time at the table without holding the attention of the players. Referees could open the scoring of this up for contribution from the Players, being conscious that some unscrupulous players may lean to awarding high marks here to help the ritualist along. (Or low marks to spite them!)
'SACRIFICE' should be scored as the HD of monster, including special ability bonuses, it would probably take to obtain the item, creature, or power sacrificed. For example if a heart of an evil dragon were being sacrificed, then the HD of the dragon it came from would be the Sacrifice score used up to a limit of the ritualist's level. Referees may wish to give some thought as to how many relics of powerful monsters they feel can be used to power rituals, I would suggest only one from each killed.
Ritual Difficulty Classes
'SIMPLE' for effects which do not impact the game greatly and are in keeping with the daily responsibilities of the caster's class and level. For example: a Cleric asking for their divine patron to bless the village harvest, reducing the chance of incidental blight or unseasonal weather; a Magic-User enchanting the broom in his study to sweep the floor clean each morning; or an Elf calling forth ancient recipes and subtle magic to bake a batch of waybread.
Simple effects require a Ritual Score of at least the half ritualist's level.
'UNREMARKABLE' for effects which have game effects, but would not seem strange for a caster of the ritualist's class and level to accomplish. For example: a Cleric blessing a graveyard or crypt so that bodies laid to rest within cannot be raised as undead so long as regular prayers are said in the adjoining temple; a Magic-User enchanting a writing desk, pen and ink to copy any book laid upon it into a provided blank tome; or an Elf performing a ritual around the boundary of their forest home which enlivens the trees to whisper a warning to the elves when non-elf humanoids cross the threshold.
Unremarkable effects require a Ritual Score of at least the ritualist's level.
'EXTRAORDINARY' for effects which are at the upper limits of what a caster of their level could expect to accomplish, and have significant in game effects, any creation or effect which is permanent without intervention is likely to require an extraordinary casting. For example: a Cleric blessing a reliquary or holy icon to allow non-Clerics wielding it to Turn Undead as a Cleric of the ritualists' level; a Magic-User creating a summoning circle which bars summoned spirits from escape unless they bargain services or secret knowledge to the ritualist; or an Elf enchanting a crystal flask of pure spring water to shine with the light of the dawn whenever uncovered.
Extraordinary effects require a Ritual Score of at least twice the ritualist's level.
Conclusion
Please have a go with this and let me know how you get on.
References
(1) https://www.quora.com/Do-archaeologists-have-a-tendency-to-simply-label-artifacts-of-unknown-purpose-ceremonial
(2) https://en.wikipedia.org/wiki/Gebel_el-Arak_Knife
(3) https://dungeonsdragons.fandom.com/wiki/Vancian_magic_system
(4) https://lorientrust.com/