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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 (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.

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.

5e, as it is normally played, doesn't contain many opportunities for players to permanently weaken their characters in exchange for temporary benefit. Adding this mechanic opens up possibilities for annoying player behavior.

For example: what if a player uses lots of magic in a short period of time, deliberately driving their character insane in exchange for power -- and then, when their character dies, brings in a new character who does the same thing? Essentially this player would be treating their characters as disposable, taking advantage of the ability to bring in new characters at will. This could be hard on your story and hard on morale.

What if a player uses lots of magic in a short period of time, deliberately driving their character insane so that they will have an excuse to attack the other characters? What if they say: "It's not my fault I opened a portal to the Outer Realms and got you all killed. The insanity rules made me do it!" (This has happened to me, though it wasn't specifically tied to an insanity mechanic. My campaign contained a thing that the characters must never do or it would end the world, and one player had his character commit suicide in order to do it, just to troll us.)

What if a player uses a lot of magic, for a good reason, and then it slowly sinks in that they've permanently weakened their character? What if they decide they don't want to play a permanently weakened character, and they lose interest in your game as a result? (This has happened to me: I added an insanity-magic mechanic to a 3.5e campaign I was running, one player started using it, and then just as his character was about to go insane he vanished from my campaign and stopped responding to emails.)

I haven't played Call of Cthulhu, so I don't know how that system normally handles this sort of thing. My guess is that you develop behavior norms, and when someone violates those norms you remove them from the game. The worry is that your D&D players might not have the same behavior norms, so they might be more likely than normal to behave annoyingly if you suddenly expose them to insanity-for-power tradeoffs.

Of course, if you have good players, they will roleplay their characters' desire to not go insane, and you'll probably be fine.

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 (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.

5e, as it is normally played, doesn't contain many opportunities for players to permanently weaken their characters in exchange for temporary benefit. Adding this mechanic opens up possibilities for annoying player behavior.

For example: what if a player uses lots of magic in a short period of time, deliberately driving their character insane in exchange for power -- and then, when their character dies, brings in a new character who does the same thing? Essentially this player would be treating their characters as disposable, taking advantage of the ability to bring in new characters at will. This could be hard on your story and hard on morale.

What if a player uses lots of magic in a short period of time, deliberately driving their character insane so that they will have an excuse to attack the other characters? What if they say: "It's not my fault I opened a portal to the Outer Realms and got you all killed. The insanity rules made me do it!"

What if a player uses a lot of magic, for a good reason, and then it slowly sinks in that they've permanently weakened their character? What if they decide they don't want to play a permanently weakened character, and they lose interest in your game as a result? (This has happened to me.)

I haven't played Call of Cthulhu, so I don't know how that system normally handles this sort of thing. My guess is that you develop behavior norms, and when someone violates those norms you remove them from the game. The worry is that your D&D players might not have the same behavior norms, so they might be more likely than normal to behave annoyingly if you suddenly expose them to insanity-for-power tradeoffs.

Of course, if you have good players, they will roleplay their characters' desire to not go insane, and you'll probably be fine.

5e, as it is normally played, doesn't contain many opportunities for players to permanently weaken their characters in exchange for temporary benefit. Adding this mechanic opens up possibilities for annoying player behavior.

For example: what if a player uses lots of magic in a short period of time, deliberately driving their character insane in exchange for power -- and then, when their character dies, brings in a new character who does the same thing? Essentially this player would be treating their characters as disposable, taking advantage of the ability to bring in new characters at will. This could be hard on your story and hard on morale.

What if a player uses lots of magic in a short period of time, deliberately driving their character insane so that they will have an excuse to attack the other characters? What if they say: "It's not my fault I opened a portal to the Outer Realms and got you all killed. The insanity rules made me do it!" (This has happened to me, though it wasn't specifically tied to an insanity mechanic. My campaign contained a thing that the characters must never do or it would end the world, and one player had his character commit suicide in order to do it, just to troll us.)

What if a player uses a lot of magic, for a good reason, and then it slowly sinks in that they've permanently weakened their character? What if they decide they don't want to play a permanently weakened character, and they lose interest in your game as a result? (This has happened to me: I added an insanity-magic mechanic to a 3.5e campaign I was running, one player started using it, and then just as his character was about to go insane he vanished from my campaign and stopped responding to emails.)

I haven't played Call of Cthulhu, so I don't know how that system normally handles this sort of thing. My guess is that you develop behavior norms, and when someone violates those norms you remove them from the game. The worry is that your D&D players might not have the same behavior norms, so they might be more likely than normal to behave annoyingly if you suddenly expose them to insanity-for-power tradeoffs.

Of course, if you have good players, they will roleplay their characters' desire to not go insane, and you'll probably be fine.