@sardonicmonkey yes, but you can’t make a pact with the Great Old Ones and not have negative consequences in your life. You just can’t. Lovecraft said so and he basically invented the GOO. It’s not about balance, it’s about respecting how awe-inspiring and terrifying these beings really are.
I'd just like to point out that this is just ONE interpretation of ONE great old one and it clearly disagrees with the general text in the PHB indicating that there are a wide range of great old ones. In your game, as DM, you can do anything you want with patrons and gods. However, you need to make this clear to players before they create their characters since they may not immediately associate ALL warlocks with madness, darkness, evil and other related effects.
Here is what the PHB says again:
"Your patron is a mysterious entity whose nature is utterly foreign to the fabric of reality. It might come from the Far Realm, the space beyond reality, or it could be one of the elder gods known only in legends. Its motives are incomprehensible to mortals, and its knowledge so immense and ancient that even the greatest libraries pale in comparison to the vast secrets it holds. The Great Old One might be unaware of your existence or entirely indifferent to you, but the secrets you have learned allow you to draw your magic from it."
Where does it say that any character making a pact with a great old one MUST have negative consequences in their life. Feel free to play Call of Cthulu if you expect all warlocks to be Cthulu followers and suffer from madness but D&D 5e specifically describes a much wider range of possible patrons, some of which "might be unaware of your existence or entirely indifferent to you".
In any case, from a playing perspective, the DM and the player should be on the same page regarding how patrons work in that game world before the characters are created so that the player has the opportunity to buy into whatever vision the DM has in mind.
To the OPs original question about how to "punish" a character ... there are either roleplay or mechanical punishments that can be imposed if you really want to.
- roleplay effects: disrupted sleep, nightmares, creepy waking visions, unusual feelings, feelings of compulsion, unexpected out of character anger/hatred/dread, sudden mood swings ... the DM can describe all of these however they want and the player can either run with it or ignore it. (As a player, if it became too persistent I would just ignore it since it is a role play element and the DM doesn't dictate how the character is roleplayed.)
- mechanical effects: levels of exhaustion due to lack of sleep, decreased spell DC/attack modifiers with spells or cantrips from the patron, gate class abilities behind an ability check - if you fail the ability check you can't use the feature, gate spells behind ability checks, if the character continues to "disobey" their patron then maybe the patron steps in preventing spell or hit point recovery on rests ... however, most of these are NOT consistent with how warlock powers are described. The character gains knowledge from the pact and are free to use it however they see fit, there is no requirement for an ongoing connection to retain these powers and abilities so the biggest mechanical effect consistent with the description might be to prevent the character from taking any more levels of warlock because their patron has decided not to share any additional knowledge. (As a player, if the DM imposed mechanical effects to force the character to make certain choices, and I didn't find it fun or interesting, I would just quit the game since very few players enjoy being rail roaded).
Positive mechanical effects for characters that do things the patron asks them could include enhanced spell casting, enhanced spell lists or additional homebrew class features.
I think the most important point from the PHB is the following:
"Work with your DM to determine how big a part your pact will play in your character's adventuring career."
The role of the patron and the pact and how much of an impact it has on the character is a decision made jointly by the player and the DM working together which is why I personally don't like the punishment approach in which a DM enforces their vision of what a pact means on a player who may consider it differently. From the rest of this thread, the player involved appears to be unaware that this is coming and may have quite a different view of the role of their patron in their character development.
I'd just like to point out that this is just ONE interpretation of ONE great old one and it clearly disagrees with the general text in the PHB indicating that there are a wide range of great old ones. In your game, as DM, you can do anything you want with patrons and gods. However, you need to make this clear to players before they create their characters since they may not immediately associate ALL warlocks with madness, darkness, evil and other related effects.
Here is what the PHB says again:
"Your patron is a mysterious entity whose nature is utterly foreign to the fabric of reality. It might come from the Far Realm, the space beyond reality, or it could be one of the elder gods known only in legends. Its motives are incomprehensible to mortals, and its knowledge so immense and ancient that even the greatest libraries pale in comparison to the vast secrets it holds. The Great Old One might be unaware of your existence or entirely indifferent to you, but the secrets you have learned allow you to draw your magic from it."
Where does it say that any character making a pact with a great old one MUST have negative consequences in their life. Feel free to play Call of Cthulu if you expect all warlocks to be Cthulu followers and suffer from madness but D&D 5e specifically describes a much wider range of possible patrons, some of which "might be unaware of your existence or entirely indifferent to you".
In any case, from a playing perspective, the DM and the player should be on the same page regarding how patrons work in that game world before the characters are created so that the player has the opportunity to buy into whatever vision the DM has in mind.
To the OPs original question about how to "punish" a character ... there are either roleplay or mechanical punishments that can be imposed if you really want to.
- roleplay effects: disrupted sleep, nightmares, creepy waking visions, unusual feelings, feelings of compulsion, unexpected out of character anger/hatred/dread, sudden mood swings ... the DM can describe all of these however they want and the player can either run with it or ignore it. (As a player, if it became too persistent I would just ignore it since it is a role play element and the DM doesn't dictate how the character is roleplayed.)
- mechanical effects: levels of exhaustion due to lack of sleep, decreased spell DC/attack modifiers with spells or cantrips from the patron, gate class abilities behind an ability check - if you fail the ability check you can't use the feature, gate spells behind ability checks, if the character continues to "disobey" their patron then maybe the patron steps in preventing spell or hit point recovery on rests ... however, most of these are NOT consistent with how warlock powers are described. The character gains knowledge from the pact and are free to use it however they see fit, there is no requirement for an ongoing connection to retain these powers and abilities so the biggest mechanical effect consistent with the description might be to prevent the character from taking any more levels of warlock because their patron has decided not to share any additional knowledge. (As a player, if the DM imposed mechanical effects to force the character to make certain choices, and I didn't find it fun or interesting, I would just quit the game since very few players enjoy being rail roaded).
Positive mechanical effects for characters that do things the patron asks them could include enhanced spell casting, enhanced spell lists or additional homebrew class features.
I think the most important point from the PHB is the following:
"Work with your DM to determine how big a part your pact will play in your character's adventuring career."
The role of the patron and the pact and how much of an impact it has on the character is a decision made jointly by the player and the DM working together which is why I personally don't like the punishment approach in which a DM enforces their vision of what a pact means on a player who may consider it differently. From the rest of this thread, the player involved appears to be unaware that this is coming and may have quite a different view of the role of their patron in their character development.