Slow Ritual Magic

This is a magic system—or, really a spell system—designed for OSR games; it assumes you're using something with modifiers (like Knave) and de-emphasized classes (also like Knave), but can be hacked around pretty easily. Only minimally-tested thus far.

It draws inspiration in part from this Ben Milton tweet and also this part of the Diablo 4 trailer (up until about 3:20), but here are the basic design goals:

1. Spells should be slow, but punchy. They should feel like the centerpiece of an encounter.
2. Spells should use both Intelligence and Wisdom, and don't have levels; this lets casters feel different to each other based on stats and materials available, rather than class packages. 
3. Spells and their casting should have some depth to them; they should be involved, engaging, and factor into the tactics of OSR-style play well.
4. Spells should feel ritual; they should be arcane and complicated and weird and specific and kind of otherworldly.

Quick notation key: STR is Strength score, [STR] is Strength modifier.

SPELLS KNOWN

This is the most-adjustable of things on here, but as a basic rule of thumb, a caster should know about as many spells as they have levels. A caster can cast [WIS] spells per day; if you don't have modifiers, call it WIS - 10 times per day or something. It should be fewer spells than normal, probably, and it should scale off of WIS, but you can adjust it past that.

CASTING

All spells have rituals; these are complicated magical procedures that are required to get the magic itself to behave in the way you want. 

Rituals

To cast a spell, a caster must start the spell's ritual: when it's complete, the spell fires. Every spell has a clock (a la Blades) that represents the steps of the ritual; when the clock is filled, the spell fires. By default, a spell's ritual clock has 12 segments, and thus has 12 steps to complete the ritual. 

I assume that rituals take up physical space roughly equal to one square, where you've got a circle of salt and candles and whatnot. To participate in the ritual, you have to be next to the ritual circle. 

Basics of Casting

On their turn, a caster can use their full action to perform steps of the ritual. To do so, they make an INT check. Before they roll, they declare how many steps of the ritual they're attempting to perform, which determines how fast the clock fills, but also alters the DC of the check:

• To fill 1 segment, the DC is 5
• To fill 2 segments, the DC is 10
• To fill 3 segments, the DC is 15
• Certain powerful magicians—like liches, exalted archpriests, witch matriarchs, and very-high-level PCs—can fill 4 segments with a DC 20 check, or possibly even more

(If you're using roll-under stats, make 2 segments a standard INT roll; 1 segment gives you +5, and 3 segments gives you -5. That math isn't 100% the same as it is with modifiers, but it's close enough.)

The caster decides how many segments, and thus which DC, they're aiming for ahead of time for each roll. If they fail the roll, they erase one filled segment of the clock. 

Just to be clear: if you declare that you're making a DC 10 check for two segments and then roll a 7, that's a failure—you can't say that you were actually trying for DC 5. Similarly, if you declare that you're making a DC 5 check for one segment and then roll a 13, you can't say you were actually trying for DC 10. What you declare before you roll is final.

Failing the Ritual

If the ritual goes more than 1 minute without any segments being filled, the spell fails, but does not misfire. If a filled segment segment must be erased but there are no filled segments, the spell fails and misfires. 

Basically, this means that you can cook a spell for a few minutes, sure, but you can't keep it at 11/12 segments filled for an hour until the bad guy shows up. Once the ritual starts, it has to finish.

Taking Damage

If a caster takes damage in the middle of casting the ritual, they need to make a CON save against a DC of the damage taken (or otherwise save vs. big physical pressure, modified by quantity of damage taken). On a failure, erase one filled segment; if it was really big walloping damage, erase two.

Firing Early

If a caster wishes, they can attempt to fire a spell before the ritual is complete, before the clock is filled. To do so, they must make an INT check with a DC of 15 + empty segments remaining. On a success, the spell fires as normal; on a failure, it fails and misfires. 

Overcharging the Ritual

After the ritual is complete and the clock is filled, the caster can choose to continue the ritual to overcharge the spell, which means they add additional filled segments to the clock by making INT checks as normal. Spells have different effects based on overcharged segments; these vary on a spell-by-spell basis. 

When a caster fires an overcharged spell, they make a WIS check equal to 10 + overcharged segments filled. On a success, it works, and fires with overcharge benefits. On a failure, the spell still fires, but only in its regular, non-overcharged form—and it misfires as well.

ACOLYTES

People other than the primary caster of the spell can assist with the ritual of the spell; these people are called acolytes. No spell requires acolytes, but all spells are helped by them. Acolytes can be fellow PCs or NPC hirelings either, but they have to be sentient humanoid-ish creatures—so your ranger's dog probably can't help, but your clever chimpanzee buddy probably can. 

Aiding the Ritual

When an acolyte is at the ritual space, they can use their action to perform the ritual, just like the primary caster. They make an INT check as normal, but can only make a DC 5 check to fill 1 segment or a DC 10 check to fill 2 segments.

If an acolyte fails their check, they erase one filled segment, as normal—which can cause the ritual to fail, as normal, too. 

Taking Damage

If an acolyte takes damage while performing the ritual, they don't have to make a save and never clear any segments. Acolytes are part meatshield, after all; their will is not bound to the spell in the same manner.

Firing the Spell

If an acolyte fills the last segment of the ritual clock, the primary caster chooses: the spell can fire then and there, on the acolyte's turn, OR the ritual can move into being overcharged. If the caster's down and out, the spell fires on the acolyte's turn. 

To count as an acolyte (as per a spell's effect), the acolyte must have participated in the spell's casting within the last round before the spell fires. If an acolyte contributed one segment right at the beginning and then took off, for example, they don't count. 

Overcharging and Firing Early

Acolytes can overcharge spells by making INT checks as normal, but only the primary caster can fire an overcharged spell. Acolytes cannot attempt to fire a spell early. 

MATERIALS AND CONDITIONS

Materials are the physical objects and things required to perform the spell's ritual. They're usually somewhat rare, expensive, and specific. Materials are usually used by the primary caster, and can usually be reused again later.

Conditions are the conditions under which the ritual must be performed. Generally, the conditions must be present for the entirety of the ritual; they stop being present halfway through, they don't count.

If you try to cast the spell without the necessary materials or conditions, you lose the spell and nothing happens. If a material or condition is present for the start of the ritual but then is gone midway through (like a torch is lit, or your bag of salt stolen), the spell fails and misfires. 

Ritual Accouterments

While all spells have specific ritual materials and conditions, there are known-magical quantities of each that can add to any given ritual. Here is a non-complete list of possible accouterments to your ritual, but work with your GM to come up with some more:

• Big fat yellow candles
• Burnt incense
• Glyphs and runes drawn in chalk
• Magical implements made of silver
• Circles of salt or sand
• A notebook or scroll covered in arcane scrawlings
• An auspicious bit of an spooky animal (like adder's fork and blind-worm's sting)
• Twilight
• The witching hour
• A circle of standing stones
• A consecrated or desecrated location (or otherwise the site of great good or evil)
• The full moon, or no moon

If three or more of these are present before the ritual starts (so candles are lit and circles drawn, not in the midst of being set up), the ritual clock starts with 1 segment filled in. For each acolyte that is present, an additional three accouterments can be used for 1 segment, up to a maximum of six segments total. (So to get to six segments already filled in, you'd need yourself, five acolytes, and 18 ritual accouterments.)

However, if, at any point over the course of the ritual, an accouterment is lost, one segment is erased, and then every three accouterments erases another. (Basically, 3 accouterments = 1 segment, but round down; if you would be at 2.66 segments, that's only 2 segments. 0.66 segments is zero.) 

Lost accouterments can include snuffed candles, broken circles, the shattering of stones, or the rising of the sun—whatever disrupts the accouterment itself. This can, of course, cause the spell ritual to fail.

A MAGICAL MISFIRE TABLE

You can use basically whatever magical mishap table you want, but here's a decent (if kinda goofy) one:

1d20	Effects
1	1d20 randomly-determined limbs in the vicinity transform into tentacles.
2	One eyeball from every living thing in the vicinity bursts. Just, like, pops.
3	The skin of everything in the vicinity turns bright green.
4	2d6 randomly-determined people in the vicinity are transformed into pigs.
5	1d6 grandparents of people in the vicinity die suddenly of a heart attack.
6	Every article of clothing in the vicinity suddenly bursts into flames.
7	A huge acid spider with thirteen eyes bursts up from under the floor.
8	You lose one of the following four senses: sight, sound, smell, touch.
9	Tattoos of curses, blasphemies, and unholy symbols appear all over your skin.
10	All of your teeth, hair, and nails fall out instantaneously, or else begin growing uncontrollably fast.
11	The brains of everyone in the vicinity shift one person to the left.
12	You age or de-age 1d4 × 10 years.
13	Every humanoid orifice in the vicinity hisses out hot steam when opened.
14	In the vicinity, all wood turns to stone, all stone turns to flesh, and all flesh turns to wood.
15	The weather changes sharply: hail, lightning, hurricane winds, ice, etc.
16	The ghost of someone either you loved or someone you hated appears.
17	A portal to another dimension rips open nearby.
18	A geyser of steam, lava, oil, milk, wine, or ink bursts open.
19	1d12 strangely-colored new stars appear in the sky.
20	Nothing.

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EXECUTIVE SUMMARY (tl;dr)

1. Spells are rituals, which have a 12-segment clock that needs to be filled in to fire.
2. On their turn, a caster makes an INT check to fill segments: DC 5 is one, DC 10 is two, DC 15 is three. If the caster fails, erase a segment. If the caster gets hurt, save or erase a segment.
3. If the ritual goes a minute or more without any segments filled, the spell fails. If a segment needs to be erased and there are none to be erased, the spell fails and misfires. 
4. After the clock is filled, the caster can overcharge by continuing to fill segments. To fire an overcharged spell, the caster makes a WIS check, DC 10 + overcharged segments.
5. To fire a spell early, before the clock is filled, make a WIS check equal to 15 +  unfilled segments.
6. Acolytes can help fill a clock, but only at DC 5 and DC 10, not 15. They can get hurt as needed.
7. Acolytes can only fire a spell if the primary caster lets them; they can overcharge, but can't fire an overcharged spell. They can't fire early. 
8. Materials are physical objects for the ritual; conditions are surrounding circumstances. Both are necessary.
9. Accouterments are generic materials and conditions; three of them present fills in 1 segment for free, but if they're lost, that segment is erased. 

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I decided to split off the spells from this post because it's getting long, but you can find Six Slow Ritual Spells here. As always, you can and should hack and remix all of the stuff here. And let me know if you get a chance to try it out!