Formula Casting

The rules for formula spellcasting presented here have been taken from Sandy Peteresen's Cthulu Mythos supplement. All credit goes to him and his team.

FORMULAS

Certain spells in this chapter have a special tag: formula. Such a spell draws upon universal principles that allow them to function even for non-spellcasters. Formula magic uses science and mathematics beyond most mortal understanding to power the casting of highly specialized spells. Most formulas are held only in shunned books and in the minds of dangerous eccentrics who know more than is safe about the secrets of reality. Generally, these spells depend upon knowledge of the alien sciences, extradimensional mathematics, and incomprehensible iconography of the cosmic forces underlying the Mythos.
A spell with the formula tag can be cast normally, as a formula, or (if it also has the ritual tag and the character has a feature to allow casting rituals) as a ritual. Casting a spell as a formula takes the normal casting time but it carries a risk of failure (see Formula Ability Checks, below). Casting a spell as a formula doesn’t expend a spell slot, so it can’t be cast at a higher spell level. Formula spells can be learned and cast by anyone with the appropriate tools and knowledge. Even characters without the Spellcasting feature or Pact Magic feature can learn and cast spells as formulas. While learning a formula spell as a normal spell requires that spell to be on the character’s class list, learning it as a formula uses alternative rule open to any character (see Learning Spells Exclusively as Formulas, below). Once a character learns a formula spell as a normal spell, the character can cast it as a formula without preparing it but must prepare to cast it in other ways. Characters with the Spellcasting or Pact Magic trait might want to learn a formula exclusively as a formula and accept the riskier means of casting it in order to avoid the cost of scribing the spell into a spellbook or counting it toward a limited number of spells known.

LEARNING SPELLS EXCLUSIVELY AS FORMULAS

A formula spell can be learned exclusively as a formula. Learning it this way doesn’t require having the spell on the character’s class list or even having the Pact Magic or Spellcasting feature. When learned this way, the spell can’t be cast normally (using a spell slot) or as a ritual but can be cast as a formula. It also doesn’t need to be scribed into a spellbook and doesn’t count against any limited number of spells known the character might have.
Learning a formula spell exclusively as a formula requires a variable amount of time depending on how it is learned. If the formula is learned from hidden clues or from independent research into alien sciences, the process takes 10 days per level of the spell. Learning from detailed instructions (as included in many Mythos tomes) takes 4 hours per level of the spell. In either case, the researcher must succeed at one of the formula’s ability checks (researcher’s choice of which). Failing the check means the secrets of the formula elude the researcher’s understanding, though the researcher can try again.

FORMULA CASTING TIME

When you cast a spell as a formula, its casting time requires ability checks made as part of the action to continue casting the spell.
Phases. The formula’s casting time is broken into five phases. At the end of each phase, you must attempt an ability check (see Formula Ability Checks, below) to exploit the unfathomable universal laws that the spell’s magic hinges upon. The first phase of a formula is usually the creation of an eldritch circle or other diagram. This phase’s ability check is labeled “(diagram)”. While not always in the shape of a circle per se, proper symbols, shapes, and glyphs are typically required.
Pausing a Formula Casting. You can pause the casting of a spell as a formula to engage in combat or take other actions, though not without consequences. If you are the primary caster, you can pause the formula casting as a bonus action. You must still maintain concentration on the spell but can take other actions until you resume the spellcasting as an action. For each round a formula is paused in this way, the DCs of all the formula’s subsequent ability checks increase by 1. The formula’s casting time does not elapse while paused.

COMPONENTS

Features and other effects that remove required components from spells do not apply to formula spells unless they explicitly say so (or the GM rules otherwise). Re-quired material components commonly include specific, unusual items. Formulas frequently require material sacrifices of blood or other vital substances. Edible components must generally be consumed during the casting. Tomes are important material components to many formulas, but whether they are mandatory or simply helpful varies from formula to formula.
New Components. In addition to common sorts of spell components, a spell with the formula tag often has other more unusual requirements you must meet to cast it. These components are diagram, location, secondary caster, and time.
Optional Secondary Casters. When you start casting a spell as a formula, you can choose to include any number of secondary casters up to any limit listed in the spell’s components line. Extra secondary casters can help in one of two ways, in addition to any benefit mentioned in the spell. They can make ability checks for the spell if they bring different skills to the table, and they can pad the number of secondary casters so that some can quit or lose concentration without ruining a spell that requires a minimum number of secondary casters.

DIAGRAM (D)

If a spell calls for a diagram, it is described parenthetically. Diagrams generally must encircle you or the target as indicated in the spell and cannot be moved once inscribed, even if the spell’s casting is not yet complete.

LOCATION (L)

Some spells must be cast at the indicated location; they automatically fail if you cast them elsewhere. Each includes a description for its component, which might be very specific (such as one particular hill) or general (such as “open to the sky” or “an underground chamber”).

SECONDARY CASTER (SC)

If a formula spell allows the participation of secondary casters (often called “acolytes”), SC is immediately followed by a parenthetical that details any maximum or minimum number of secondary casters required to cast the spell. If a spell description has no secondary caster component, that spell does not permit the assistance of secondary casters. While secondary casters can help by attempting the ability checks the primary caster assigns them, their chief purpose is to use their action each turn to continue the spell’s casting (which precludes other actions, including the Help action).
To join a formula spellcasting as a secondary caster, you must use your action to provide somatic or verbal components after the primary caster begins casting the spell but before the primary caster’s next turn. You need not know the spell or understand how it works in order to assist, although you must be able to follow instructions from the primary caster if you don’t know the spell. The primary caster can provide these instructions as part of the action to continue the spellcasting or explain beforehand. To be a secondary caster, you must maintain concentration on the spell just like the primary caster. Unless stated otherwise in the spell description, secondary casters must be within 100 feet and have line of sight to the primary caster and each other during the entirety of a spell’s casting.
If a variable number of secondary casters is listed, the participation of any secondary casters beyond the minimum is optional. If the minimum is 0, all are optional. If you start with more than the minimum number of secondary casters involved, optional secondary casters can stop partway through (perhaps to defend the group from danger or because they were killed) without spoiling the spell. Any benefit the secondary casters provided is lost when they cease acting as secondary casters. If you were a secondary caster, you cease to be one when your concentration ends, when you end your turn more than 100 feet from or out of line of sight of the primary caster, or when your turn ends without you using your action to continue casting the spell. If these circumstances reduce the number of secondary casters to below the spell’s minimum, the spell immediately fails but no spell slot is expended even if the spell was cast normally.
Secondary casters don’t have to spend their actions continuing the spellcasting or remain within 100 feet or within line of sight of the primary caster while the spell’s casting time is paused.

TIME (T)

Some spells automatically fail unless you cast them at a particular time listed parenthetically in the spell’s components line. Time components are often astrological and require one or more stars or planets to be visible.

FORMULA ABILITY CHECKS

Casting a spell as a formula requires special ability checks as part of the action to continue casting the formula. Although a formula always requires three successes, often using different skills, one or two failures along the way won’t ruin the spell. A formula’s required ability checks are listed after the spell’s components. One of the casters—either the primary caster or a secondary caster (see above) the primary caster specifies—attempts an ability check at the end of each phase of the casting time. The primary caster can choose a different caster at the end of each phase. These checks can’t be made as a passive check. Unlike other ability checks, each individual success or failure has no effect by itself. Keep track of successes and failures until you collect three successes or three failures. The number of both is reset to zero when the formula casting ends, whether in a successful casting or not. Each time an ability check is called for, it must be the first ability check listed that the casters have not yet succeeded on. On your third success, the formula is complete, no further ability checks are required to continue casting the spell, and the spell takes effect successfully when you finish the casting time. On your third failure, the formula fails, the spellcasting ends immediately, and you and any other casters experience the formula’s failure consequences. Regardless of how the formula ends, all participants experience backlash.
     Rolling a 1 or 20. When you make an ability check to cast a spell as a formula and roll a natural 1 on the d20, you (but not other casters) get disadvantage on the next ability check you make to cast that spell. If you roll a natural 20 on the d20, you (but not other casters) get advantage on the next ability check to cast that spell.

DIAGRAM CHECK

A formula that calls for a diagram has an ability check listed as “(diagram)”. This is the ability check to create the diagram component of the spell. Having an example of the diagram on hand (as is included in most Mythos texts containing a formula spell) gives you advantage on this ability check.

SPELLCASTERS

If you have the Pact Magic or Spellcasting feature, your understanding of the fundamentals of magic informs your work. Add half your proficiency bonus to an ability check to cast a spell as a formula if you don’t have proficiency in that skill. If a formula ability check adds your proficiency bonus, you add one and one-half times your proficiency bonus to the ability check instead.

MYTHOS TAINT

If you have been tainted by cosmic forces, you gain a bonus on any ability check to cast a formula other than the diagram phase ability check. The bonus is proportional to the severity of the taint. The GM makes the final ruling on the severity of the taint, but as a general guideline, having a distant aberration ancestor or any dread, frightened condition, or short- or long-term madness caused by Mythos phenomena gives a +1 bonus. Belonging to a race linked to one of the Great Old Ones or other cosmic forces, or having an insanity or indefinite madness caused by Mythos phenomena, gives a +2 bonus. Being a chosen vessel of a Great Old One or other cosmic force, or having multiple insanities caused by such a force, gives a +3 bonus. These bonuses do not stack; only the highest bonus applies.

SAVING THROWS

When you cast a spell as a formula, the DC for any saving throw the spell allows (if any) is equal to 8 + the primary caster’s proficiency bonus + the primary caster’s Intelligence, Wisdom, or Charisma modifier (whichever is highest).

BACKLASH

Whether it succeeds or fails, any spell cast as a formula exposes the caster’s mind to multiversal concepts beyond mortal limits. Normally, the primary caster takes 1d4 psychic damage per level of the spell and each secondary caster takes 1 psychic damage per level of the spell. Some formula spells specify a different backlash. If the spell has a duration that requires concentration, backlash damage doesn’t risk causing any involved casters to lose concentration on it.

Failure

When you fail to cast a spell as a formula (whether as a primary caster or secondary caster), you unleash unfathomable forces that take a dramatic psychic and physical toll on you and any other casters. Normally, each caster gains one level of exhaustion and becomes magically frightened of the spell’s components and any associated texts and creatures until finishing a long rest. If your game uses the dread rules, the save DC of the three levels of dread gained instead of this frightened condition is equal to 10 + the level of the spell. Some formula spells specify a different failure consequence.

Hopelessly at bay, weaponless, and knowing that any show of physical violence would bring a score of attendants to the doctor’s rescue, Joseph Curwen had recourse to his one ancient ally, and began a series of cabbalistic motions with his forefingers as his deep, hollow voice, now unconcealed by feigned hoarseness, bellowed out the opening words of a terrible formula.
“Per Adonai Eloim, Adonai Jehova, Adonai Sabaoth, Metraton...”... the dogs in the yard outside began to howl, and … a chill wind sprang suddenly up from the bay...

—H. P. Lovecraft, The Case of Charles Dexter Ward