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Cthulhu d20 has a magic system in which spells aren't tied to classes nor to levels. Anyone can learn and use a spell, if they're willing to pay the price, which usually comes in the form of a temporary or permanent ability loss (drain) plus a loss of Sanity points, besides whatever in-game consequences the DM sees fit.

This system, in my opinion, would work extremely well fluff-wise in a dark fantasy D&D 5e world as well — such as Ravenloft / Curse of Strahd, for example — either as an add-on besides the standard magic system or as a full replacement of that system. Let's focus on the previous, though:

What are the most obvious pitfalls of introducing the magic system of Cthulhu d20 into D&D 5e as an add-on, making cost-based spells available to practically anyone who has the chance to learn them in-game; and what are the best ways to avoid those pitfalls when integrating the two systems?

Would such a merger break the game? What steps and rule tweaks would ensure the smoothest possible extension of the core D&D 5e system? (Sanity would have to be used, obviously, as per the 5e DMG.)

Note, please, that there's another similar question out there on the site (which was also asked by me :)) about 3.x and the same Cthulhu d20 system. However, what I'm asking here is not the same (though even if I did that, the answer would have to be different, considering the difference between 3.x and 5e.) What I'm asking here is how to best port the complete spells section of CoC d20 into 5e as an addendum to work alongside the official magic rules, which would not be affected by the levelless cost system. Please, detail the major pitfalls your proposed solution helps avoid. (Nonetheless, it would be interesting to see a wizard who used both the leveled, costless spells and the levelless, costly ones. :))

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    \$\begingroup\$ Answers should keep RPG.SE's "good subjective" guidelines in mind, and avoid untested speculation in favour of solutions based on experience with situations similar to the asker's challenge. \$\endgroup\$ – BESW Apr 14 '16 at 13:23
  • \$\begingroup\$ I don't have enough experience with CoC to say whether this would help, but "levelless, cost-based [effects]" do exist in 5e as "bargains with devils" (or hags, angels, etc.). You may want to use that as a useful power-gauge. \$\endgroup\$ – Cireo Nov 25 at 22:35
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My background: I've run some adventures using insanity themes. I once had a character who used insanity-magic and was about to go insane, when his player decided the campaign was too dark for him and stopped showing up at my table. I once had a character who used insanity-magic and went insane on purpose because he wanted to kill the party and destroy the world. Here's what I would do in future if I wanted to use insanity themes again.

I think you should start by looking at the Same Page Tool.

D&D 5e is (usually) about a group of heroes who work together to save the world. Call of Cthulhu is (usually) about a group of normal people who stumble into eldritch horrors, go insane, turn on each other, and die horribly. These are very different games. Which game do you want to be playing?

Suppose that a player deliberately drives their character insane, then uses that insanity as justification for killing the party and unleashing eldritch horrors on the world. Is that: (1) totally appropriate and in-genre, or (2) annoying because that player ruined everyone else's fun?

Suppose that the party as a whole embraces the use of insanity magic, becoming more and more dysfunctional and evil until eventually the group shatters and everyone dies. Is that what you wanted from the adventure? Or do you want something more along the lines of "people go a little evil for flavor, but not so much that it makes them lose"?

Decide in advance what sorts of actions are "in-genre" for your game, then make sure you and your players are in agreement about that. Once you've all decided what you want your game to look like, you'll have a much easier time getting it to look like that.


In terms of mechanics: the problem you're going to have is that D&D doesn't really do "permanent damage". Most reduction to ability scores goes away after a short or long rest. Even if you house-rule that certain spells cause permanent damage, the greater restoration spell can cure that stuff.

A lot depends on what your players will tolerate. If they're on board with it, you could improvise some sort of Sanity Points system for representing permanent sanity damage. Most players don't like having their character's impending insanity hanging over their heads, though. This could cause them to lose attachment to their character, or it could cause them to lose interest in your game.

I think a more awesome approach would be to impose narrative costs: when you use the forbidden magics, you take temporary Wisdom damage and you make a Wisdom check. If you fail, you release evil into the world. The evil could take the form of another monster you have to fight, or it could be that something you relied on has become foul and twisted and corrupted, or maybe it ignores your character and makes life miserable for the nearby villagers (depending, perhaps, on how badly you roll). I've done similar things in Apocalypse-World-based systems and it's been a lot of fun.

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    \$\begingroup\$ Wow--really nice rewrite. +1, and I've removed my earlier comment. \$\endgroup\$ – nitsua60 Apr 15 '16 at 0:58
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I'm not sure if you'll be able to gracefully staple the 2 systems together -- they're both quite different, with CoC placing more emphasis on weaker characters than D&D's heroes. I also think that Dan B's answer is spot on, in some ways. Including a sanity system can easily cause some players to screw around with it.

My personal experience in introducing a sanity mechanic to my 5e sessions resulted in two kinds of players: the first kind studiously avoided any possible fear, in order to avoid tarnishing his character; while the other embraced the system in order to justify his character's madness and descent into antagonism. It's the first kind of player that I'd worry about most -- they tended to disengage from the system in a way that wasn't quite in line with a lot of the table.

A possibility you can explore is tying the ideas of cost-based spells to things other than SAN, however. Thematically, casting from health, strength, or experience could provide the same sort of trade-off that you're looking to balance, though adapted in such a way that players wouldn't permanently harm their characters.

In order to balance this, however, you may want to look at having steeper short-term costs. In a sanity system, a 1 or 2 point drop might stay with you until death; the short term system will be fixed much sooner, so it makes sense to me that it ought to be proportionally more dangerous while it lasts.

Some ideas for costs that may work:

  • Casting spells from HP, at a cost of (maybe) 25% max HP per spell level
  • Casting spells inflicts debuffs that may last until the next long rest. The Poisoned condition seems appropriate here mechanically, if not thematically
  • Casting from STR or WIS, saddling them with debuffs until they are restored at the next major town
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Potions

In D&D 5e the rules for potions (located in the DMG) most closely mirror the desired effects you want. Any class can use them, but it is generally either a time-limited effect or it has some drawback (causes addiction or another of the madness conditions, requires a Con save, or causes a negative physical transformation). Also there is a list of side effects that potions have and what can occur on a failed creation attempt.

The potion mechanics can be applied to other methods of learning spells (such as forbidden knowledge from a sealed grimoire granting the character the Flesh Ward spell and causes hardened protuberances to spurt out all over their body granting them: 1d6 temporary hit points but forcing them to roll on the temporary madness table, but if they already have that madness then they roll on the permanent madness table instead)

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There may be problems with a straight import.

I'll do my best to make this answer more applicable than my first. One of the main hurdles I hit writing that one, which I believe may apply to other users, is that I'm not terribly familiar with Call of Cthulhu d20. With that in mind, I'd like to summarize the important bits we'll be dealing with. Once that's done, we can have a context in which to work with the levelless spellcasting system.

Basis

To start off with, CoCd20 has many similarities to 3.5 -- the bones of these systems are much the same. All characters have the classic 6 Ability scores, and most scores are between 8 and 20. The Ability Scores boil down to Ability Score Modifiers in the same way, and the modifiers are applied to skills and saving throws in the same way.

Differences

There are a few salient differences here. Among these: classes have been replaced with skill templates, and players can choose a defensive or offensive track for levelling. The difference is bonus to saves or full BAB scaling, so it's not too complex.
An important thing to note here is that we can usually expect differences in characters to be limited to about (ASM + 2), which may give a better idea of how things compare between characters.

However, the most important is the introduction of the Sanity mechanic.
From what I understand of d20 CoC, this is an important balancing feature, so I think it's important to understand how it works in order to contextualize the spellcasting mechanic. The other main balancing mechanic is Ability damage -- we'll get to that after Sanity.


Sanity2

Each character has a Sanity score, ranging from -10 (incurably insane) to 99 (paragon of sane). A character has a starting sanity of (5 * Wisdom1).
Maximum Sanity can vary from character to character, and is (99 - Cthulhu Mythos Modifier). In short, the more a character knows about the Mythos, the less stable they are.

When confronted or working with the Mythos, the player characters will occasionally come across disturbing things. In many cases, this results in a Sanity Saving Throw - a roll of d% vs. the character's current Sanity. The Save is successful on a result less than the character's current Sanity.
Individual events chip away at Sanity reasonably slowly -- finding a corpse is 0/1d3, and only damages on a failed saving throw, for example. A friend's death is 0/1d6; torture - 2d10; seeing Cthulhu - 1d10/d%.
Taking an amount of sanity damage that's greater than a portion of your Wisdom1 causes temporary insanity, and greater damage causes greater effects.

To give some perspective, pages 52-57 of the handbook are devoted to a vareity of psychological issues as reference for roleplaying.

This is not a d20-style system3.

Instead, it's a carryover from the earlier d00 CoC systems, and that presents some problems. My inkling is that it doesn't just carry the dice with it -- the Sanity system has a lot of inbuilt assumptions about how the characters interact with the world.

One of the most important assumptions therein is how 'heroic' the characters are. The most effective form of restoring Sanity in the handbook is sustained Psychoanalysis, which restores 1 point/week. This would be a fundamental disconnect with most D&D games I've been a part of, in which downtime is severely reduced. If you intent to import the levelless casting system without modification, you'll have to bear in mind all the assumptions that come with it -- namely , this slower-paced approach.

This definitely feels like a long-term balancing system. You can cast many spells before running up against your limit, but once you get there, it's really hard to recover.


Ability Damage

Thankfully, Ability Damage is much more straightforward than the Sanity mechanic. In short, when casting a spell, the caster needs to front a temporary sacrifice. For many spells this is minor -- a point or 3 of Ability, meaning a 1 or 2 point reduction to their Ability Score Modifier.
Some spells (e.g., Call Deity) demand a higher price -- 20 points of damage. However, the spellcasting rules allow for this damage to be mitigated over several casters4.
Additionally, some spells may require permanent reductions to Ability.

Healing Ability Damage

According to the damage information5, temporary Ability Damage heals at the rate of 1/day. This isn't too far off of what I rule-of-thumb for D&D -- about 1 point per long rest seems about right.

t really feels like the Ability Damage is not meant to be a lingering things. In a game of CoC, where recovering the Sanity cost of a single spell takes about a month, recovering 1 point per day of Ability Damage is not too bad at all.
Indeed, it seems to be balanced around short-term tradeoffs -- after most spellcasting, a character should be back in good shape after about a week.


Spell Comparisons

At this point, it seems like it may be fun to do some spell comparisons. Generally speaking, though, it seems like the spells are broadly similar -- Fear out of 5e seems similar to Cause Fear out of CoC. As balancing spells from a 3.5 environment is an entirely different essay, I'm just going to recommend using the equivalent 5e spells whenever possible.
The main differences are in how they're applied -- aside from casting costs, we're really considering the differences between how 3.5 and 5e do spells.

So, what are those costs?

Well, for Blindness/Deafness, 5e considers it to be a second level spell of the Necromancy school. For casting, it obviously takes second level slot.
CoC has the caster pay 3 Int damage and 2d6 Sanity instead of the slots.

Fear is a 3rd level illusion spell in 5e, and takes the requisite slots.
In CoC, Cause Fear takes 2 Wis and 1d4 Sanity.


So what does this tell us?

Well, the way I see it, we have two cases to work with when it comes to bringing in the spell system.

The first case covers spells that have 5e equivalents. In these cases, it's questionable. You'll need to bear in mind how characters interact with the sanity system, and play such that they have to interact with it (whether through spellcasting or not). However, I think it could be a great addition to a campaign.
You'd want to be aware of which classes might want which spells, and adapt spell costs accordingly. A rogue may be tempted to make use of Invisibility6, and the 2 Int damage might not put them off too much. Likewise, Healing Touch's7 low Wisdom and Sanity costs may lead to all non-Cleric characters becoming surprisingly competent healers.
Restricting long rests, and consistently reminding them of the limits of their sanity would be required to keep things from getting out of hand. Regardless, players might really enjoy the added freedom and power that it provides.

The second case is for spells that don't have a 5e equivalent. In these cases, it is of incredible importance that they are reevaluated and rebalanced. As an example, Flesh Ward7 grants damage resistance of 10/+1 for up to 50 points of damage. For the cost of 2 Int damage, and 1d4 Sanity, this is incredibly overpowered for fifth edition.
Bear in mind that many spells in the handbook fall into this category. In almost every case, you'll need to do a rewrite of the spell to ensure that it's even close to balanced. If you're hoping to get some guidance, it'd be useful to find a guide for converting 3.5e spells to 5e.


Final thoughts

As other answers have mentioned, you'll want to make sure that your entire group is onboard with the flavor and tone of the game. You'll want to have everyone more-or-less on the same page, and it'd definitely help to have them open to possible changes.
A modification of the spellcasting system like this is huge, and should not be underestimated. It will most definitely change the balance of the game, regardless of how well it's implemented. Bear in mind that this isn't necessarily a bad thing, it's mostly just a thing. So long as you and your group are open to experimentation and willing to find a new level, I think this could be a lot of fun.

If you ever try this out, be sure to let us know how it goes!


Footnotes

1: Note this is the Ability Score, not the modifier
2: AKA "Chapter 4"
3: Obviously
4: Multiple Casters, page 128
5: Damage, page 62
6: "Hide From the Eye", page 142 of the CoC handbook
7: Page 142 as well

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Without going into specific details, I'd consider the following:

  1. XP drain.

  2. Valuable spell components (GP).

  3. Chance of drawbacks (cursed magic items (DMG) can provide inspiration).

  4. Research of magic (abilities + time + cost).

  5. If you want to complicate things even further, you could make (some?) magic dependable on surroundings, e.g., healing only in good-aligned locations, aquatic spells only if a source of water is available, etc.

My advice would be to not mix two separate systems into one. We tried to do this with time-travel (modern-day firearms) and even with much first-hand experience on the subject (three army veterans), it's nigh impossible to do correctly.

Please also note that defences against magic should always be stronger that offensive results, otherwise game-balance will be inherently dangerous to players.

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    \$\begingroup\$ Have you experience of having done any of this yourself, or seen it used successfully by other people? \$\endgroup\$ – Wibbs Dec 19 '16 at 10:24
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If I were to implement a system such as you proposed, I would limit the spell's available to the warlock class, for 2 reasons.

1) the warlock and their spell's are already flavored appropriately for the setting you are borrowing from.

2) the mechanics of how the warlock gets her spell's are similar in many ways being granted by eldritch beings.

For the costing system and in order to keep things balanced. I would make the spell levels force a Wisdom save -1 per level of the spell. If the player fails the save, they lose their sanity (could use the fear rules as one example for how an insane player must react) for an equal number of their own turns.

This maintains balance because, if the player were to cast a 3rd level offensive spell, it may wipe out the enemy in one shot, but the down side to this is if they fail their save, they run around for 3 turns mostly out of the player's control.

In addition, you could create a consequences table to roll d20 against, using the same penalty as the will save with least level consequences being closer to 20 and worst consequences being closer to 1, lowest level consequences being fear for instance, running away, and highest level consequences being something like running to embrace your enemy, attacking your friends. Middle consequences being temporary perhaps ability score loss for a number of turns equal to the level of the spell.

This keeps everyone balanced both between all player classes, but also between players and the environment. Yes you might be lucky and be super powerful this fight, but the next you might end up going so insane that you decide that the bearded devil really just needs a hug.

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    \$\begingroup\$ Why use the fear rules for insanity when you have a Sanity ability score variant rule and rules for insanity/madness in the DMG? \$\endgroup\$ – LegendaryDude Apr 14 '16 at 13:15
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    \$\begingroup\$ Oh, I'll have to look at those. \$\endgroup\$ – Escoce Apr 14 '16 at 13:19
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    \$\begingroup\$ The Stack's "good subjective" guidelines ask us to avoid providing answers that rely on untested speculation. What experience (your own or others') can you provide to back this up? \$\endgroup\$ – BESW Apr 14 '16 at 13:20
  • \$\begingroup\$ @LegendaryDude ok so the variant rules for Madness in chapter 8 are very similar to what I am suggesting, however the sanity ability score I don't like so much. Wisdom is the usual ability to used to meter such interactions. Is the player wise enough to recognize what's going on (recognizing a bad trip for instance and just going with it until it's over) or do they go mad as a result of not being able to grasp that what's happening will eventually pass. \$\endgroup\$ – Escoce Apr 14 '16 at 13:29

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