Rituals (5e D&D )

A wizard did it

Concept: beyond a prescribed spell list, magic users (or anyone with the right books, ingredients and facility (likely a mix of Int, Wis and Cha, or just pure Cha)) can effect anything they want. The idea is to let the players and GM redefine the solution to an obstacle using magic, but at a steep cost and to add sub quests.

Rituals are broadly defined by effect. To cast down a foe, to find what is lost, to learn a true name, to learn a secret of an enemy.

Books and scrolls found in old and lost places may contain ritual instruction though wizards and the like may discover a ritual by spending as many days (or hours, if the effect is duplicative of a spell known and trivial to the caster) in contemplation.

Ritual foci are required for ritual's not confected by magic user contemplation, and then are still strongly desirable as they guarantee success. Foci tend to be macguffins, and confected rituals have pseudo foci kludged from pledges, the casters will and body.

There are three poles to consider.

1. Relative ease. The caster posits an outcome or desire. The GM considers the obstacles to obtaining the outcome naturally. Compare this difficulty to the magnitude of die sizes here and eyeball the die size. If mimicking an existing spell, use the spell's level to determine the size (sl6 is a d100,  a cantrip is d4, if greater than an sl6, add 55/additional spell level to the d100).

Divide the max die value by 2, rounding down.  This is the ritual's inertia (effectively, hp).

2. Similarity to spells. A caster with knowledge of spells or any spell - like ability may have advantage on all rolls. Dragonborn of a blue chroma may have advantage when calling thunder, aboleth and illithid and squids all gain advantage on ink, obscuring, psychic effects, etc.

3. Time and Power. Takes a day for its relative power (a d100 ritual takes seven days, a d4 one). To speed things up, add another half die to the inertia or another 2 pledges (reduces time by one step or d20 hours).

Resolution. Rituals are resolved narratively. Non casters can pledge blood (reduced to 1 ho for a day), treasure (1,000 gp, must be in jewel or statue or unworkable ore. All ancient or long lost. Hideous obelisk of Leng, reliance of the stars, etc. Coins stamped in infernal or lost mints also work), promises (enforcement of which the GM will extract in force at the worst/best time. Usually involves infernal or terrible agents.) Or  spell slots (all for the day) or an uncommon magic item (consumed or destroyed or exhausted, rarer items.counting as more pledges, but similarly depleted regardless if all pledges are necessary). 

The GM establishes the steps before casting, each step requires a contribution of the above and reduces the inertia by a die roll. Casters may also pledge as above. Pledges may be made in addition to the steps. At each step/pledge, roll a die and reduce the inertia by that much.

Steps may be opening portals or creating a jackdaw of rubbish and paper and bird bones or whispering into a bole in a frozen tree. There are as many steps as the GM wishes, usually as many as characters in the party less d4 (the point is to make things interesting). What constitutes a step is up to the GM, and the GM may, probably will, solicit suggestions from anyone casting or pledging.

Additional pledges can be made, and must be made if the ritual's inertia isn't overcome at the end of all the steps and after all the pledges are rolled or else the ritual fails.

Using a foci guarantees success in most cases though defeating inertia with rolls likely means you can keep the foci for reuse (like a catalyst).