Ok, I'm currently helping out as a assistant to our DM, (I'm not actually a character) and me and her have come up with a 'curse' that we want the main villain (The Dark Queen, a NPC) to inflict on the land that will cause any character who is with a good alignment (does not affect evil or neutral) and changes their alignment to an evil alignment. EX: chaotic Good become chaotic evil. 

 We're having some difficulties with how to invoke this curse. We have all the mechanics figured out. But our question is, would the villain have to have the 'bestow curse' spell to invoke the curse? Or does she simply need to know how to invoke the curse ? IE: have all the ingredients and items and the wording of the curse and other necessary things needed to invoke the curse. Also, we don't simply want to have a caster use the 'remove curse' spell to end it. There will be a certain thing needed to remove it. We also do not want the curse to be detected by simply using a 'detect magic' spell. The effects of the curse will have to be RPd for it to work. The curse doesn't immediately change the alignment, it slowly changes it over a period of time.

Also ,how would we inform the players who get affected by the curse. Should it be done privately or in-game? This is a PBP game. 

My advice to you is to make the curse a story element rather than getting stuck in the mechanics of the curse. I would make the curse instead a curse-like feature that is impervious to remove curse, but otherwise behaves like one. For bonus points, let a casting of remove curse provide some information about the curse or how to remove it (a quest perhaps?) That is a good way to use a mechanical tool to provide some narrative content into your game.

Here's the tricky part about something that changes a character's alignment... as much of an amazing moment it would be to shock your players with something like that, it's best to get a player's permission before forcing them to play as a completely different alignment. If I was starting a new game, and three sessions in the DM says, "Oh, by the way, something completely out of your control has flipped your character's alignment, and you're evil now", I would quit the game.

That said... as a DM (or DM's assistant, in your case), spells and magics you use don't have to follow the same rules as the spells available to players. This "Curse" needn't follow the rules of the Bestow Curse spell. If you want this spell to exist and you have planned out rules for how it works and what the PC's will need to do to deal with it. Think of various magic items that have unique abilities that don't directly correlate to any existing spells... the DM is free to invent new magic and is also free to have it not susceptible to spells for any reason. If you want an in-rules justification for that, you can think of something like Wall of Force, which states overtly that Dispel Magic simply doesn't work on it. You can just as easily add to the description of your homebrew curse that it cannot be dispelled by the Remove Curse spell.

As for informing the players... assuming everyone's comfortable with sudden alignment changes, then you should inform the players privately and let the change become revealed through their actions.

I can't agree with transmorpher enough. Don't try and force this on the players. 

And in practical terms, how would you enforce it? Tell someone they really need to be leaving the orphans in the burning building since saving them would be too good of a thing to do? Moreover, running a campaign with evil PCs is tricky, to say the least. Seems like it would be even trickier if you do it with people who didn't actually want play evil characters.

If you really want to do it, give the PCs an out. Have them be from a neighboring area, and the powers that be there have noticed this corruption and want someone to put a stop to it, then they give the PCs each some amulet or ring or whatever kind of trinket that makes them immune to the effects of the curse. Tell them that if they lose it, they'll need to start making saving throws or slowly be overcome by the evil, until they can get it back. They could even find someone they want to help and share the amulet with them, like when a scuba tank runs out of air and two people take a couple breaths each. That would put in some interesting tension. Give them something where they can play the characters they thought they signed up to play. Forcing an alignment change, not a great idea.

All good advice. Thanks. We have worked out the details pretty much, the PCs will have a way out ,we already planned that. We'll be talking with the PCs one on one to see if they are comfortable with this before we implement it. We have a pretty understanding group and are asking for some serious RP, and this would be a great way to do it. They would definately have to RP it. We have one PC that will be the KEY to everything, she cannot be affected by the curse because she has a dissadvantage that prevents her from becoming evil or doing any harm to others.

Thanks for all the advice. I shared all of this with the DM last night. We feel that everything should work out. We wanted to add a little element of surprise to liven things up a bit. Most campaigns center around fighting monsters and escaping dungeons and such, we wanted to make ours a bit more interesting and different than the norm. Our campaign relies heavily on RP and the players have been asking for more RP. We thought this would give them just that. We feel that players in a game like this should expect anything and everything and just RP it to the best of their abilities. We're going to just homebrew everything when the time comes. It will come in our next encounter. Again, thanks for all the advice. It did help us out a lot. We now know what to possibly expect when this gets implemented.

BTW, the idea came from a TV series called "Once Upon A Time". There was an episode in there that used a curse that when a person was hit with it, the curse caused them to turn against their own friends thus becoming evil. We thought that could be a very good encounter for us and would set the stage for a good story plot. Right now we've been just trying to get players to figure out who the 'bad guy' is and what their purpose is. Plus we're trying to establish groups and allies. We're throwing in some slightly easier encounters for now to see how well the players can work together to figure things out. That is when the real 'fun' begins.

Quote from Princexylan478 >>

All good advice. Thanks. We have worked out the details pretty much, the PCs will have a way out ,we already planned that. We'll be talking with the PCs one on one to see if they are comfortable with this before we implement it. We have a pretty understanding group and are asking for some serious RP, and this would be a great way to do it. They would definately have to RP it. We have one PC that will be the KEY to everything, she cannot be affected by the curse because she has a dissadvantage that prevents her from becoming evil or doing any harm to others.

 One more comment. You say that "the PCs will have a way out". This does not deal with the fact that they likely won't want a way out after the change occurs. An evil creature is selfish and unless there is something in it for them (i.e. saving their own life) they will NOT go off adventuring to find a "cure" for something that they don't think needs "curing". If the characters are played in-character with their modified alignment they will NOT want to do whatever you have come up with to change it back unless it is in some way advantageous.

In addition, anyone affected by the curse is NOT going to tell anyone else. Assuming they are modestly intelligent or wise they will hide the change and more or less act normally. They can remember what they were like, they know what the party members expect, they aren't too likely to suddenly turn into a psychotic killer (keep in mind that chaotic evil still has a sense of self preservation) and they likely aren't going to kill party members until they can get away from them.

Which brings up another issue, if role played, unless you have given the party an over riding reason that will be beneficial to each member to stick together, the ones affected by the curse may well decide that heading off on their own (so they can do what they want to do with goody two-shoes looking over their shoulder) makes far more sense than staying with the party.

So, there are several ways that this curse, applied to a party will self destruct an entire campaign unless the story has a very focused plot line that for some reason makes it more beneficial for each and every character in the party, no matter what their alignment might be, to work together.

Defeating the "evil queen" is likely not sufficient since some of the evil characters will want to join her side if she offers them enough (which she should) while others will plot to destroy the rest of the party while using them in a scheme to usurp the position of the evil queen. You can expect PVP to develop from this plot line so you should plan for that as well.

PS of course everyone may choose to play neutral characters so the entire plot line will be moot anyway which might be another complication to consider.

Quote from Princexylan478 >>

We have one PC that will be the KEY to everything, she cannot be affected by the curse because she has a dissadvantage that prevents her from becoming evil or doing any harm to others.

 This can be dangerous. D&D is a team game, hanging it all on one PC is risky. What if the character dies? What if the player moves away or otherwise stops coming?

And for the other players, it’s not very fun to be 2nd-their heroes. Their job becomes not solving the problem as much as keeping the other character alive, so that character can solve the problem. 

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Here's the tricky part about something that changes a character's alignment... as much of an amazing moment it would be to shock your players with something like that, it's best to get a player's permission before forcing them to play as a completely different alignment. If I was starting a new game, and three sessions in the DM says, "Oh, by the way, something completely out of your control has flipped your character's alignment, and you're evil now", I would quit the game.

Really ? It's a trope of many books and movies, that corruption that turns someone good evil at the stroke of a pen. Of course, it should not be abused, and it should probably be reversible somehow, but it's a great roleplay opportunity, both when the alignement is reversed, but also to deal with the consequences after that (guilt, things that you don't remember doing, changed attitude of NPCs, people not believing you, maybe even a stain on your soul). I have quite a few fond memories of sequences like that. Including one in which a character had been replaced by a kind of doppleganger, and the player was playing that but without knowing it was, just that his alignment and objectives had shifted.

Although obviously the DM should not do that to cause massive incomfort to a player, where in the rules (RAW or RAI) does it say that you need to seek permission from the player to subject his character to mind control ? Because there is not  much difference here.

 You are correct that there is no Rule that states a DM needs permission to mind control their PCs. I'm not talking about Rules. I'm simply talking about player enjoyment of the game. Players get attached to their characters and often have big plans for how they want to play and what they consider to be fun. Obviously any player joining a campaign should be aware that not everything is going to go 100% the way they plan, and there are plenty of horror stories about players who came in to games expecting everything to cater to them and the story they want for their character, and that's a huge problem. But a full good/evil flip can be a MASSIVE change that ruins the part of the game that some players are enjoying and seeking. If I came to a D&D game and was told to create a character and plan out their personality and motivations, I would feel betrayed and like my time was wasted if I'm forced to play them as a very different character.

That said, it all depends on the player. I'd imagine there are many players who will relish the roleplay opportunity or the unpredictability. Maybe there are even people who will be upset and depressed when they first learn of their alignment change but grow to love the roleplay opportunities it presents as the story progresses. I'm just saying that you have to make sure that your party is going to be receptive to something like this... just like it's important to know that your party is comfortable with things like animal abuse or suicide if you're planning to include those in your story. I actually ran into a problem in a recent game where I had a villain torturing a rabbit as an over-the-top "kick the dog" moment just to remind the players that they're meant to be villainous and to assuage their concerns about fighting them. However, I didn't realize how upsetting this was to some of my players and it made everyone miserable as the visual was so upsetting it ruined the fun of the fantasy adventure game everyone came together to play. I wasn't trying to upset my players as much as I did and we all had a conversation about it, but it was something I forced on them without realizing how it could affect their enjoyment of the game and, if I was too stubborn to change or admit fault, the players might not want to keep playing in case I were to torture more animals in front of them.

I think what would bother me about a sudden, unavoidable alignment shift would be the "Gotcha!" element of it. It gives me a sense of, "You signed up for one thing, but I'm giving you something completely the opposite", which seems unfair. On the other hand, something like Avernus is a good example of how I feel something like this can be handled more organically. If the players are aware ahead of time that going to certain places or performing certain actions risks triggering some kind of change, then it feels like the player has some agency. If I go into Avernus and a single NPC warns, "beware, the land itself will corrupt those within it", then I'm at least going in knowing the risk. It's my choice, then, if I want to play as the character I created for that particular adventure... maybe I spent a lot of time creating a character with clear morals and personality that I don't want to see corrupted, and in the extreme case I would ask if I can play as a different character, but most likely I'd be willing to take the risk if I know the risk is there.

I know that examples on this site and others often go to extremes. There are certain whiny, self-entitled players who refuse to be charmed or controlled in any way, but in the same vein, there are also DM's who abuse mind control and charm spells to force players to force players on a railroad (which, to be fair... I don't think railroading is as bad as people make it out to be, but it's still a contentious topic) or, much worse, use those kinds of spells to do heinous things. I've known people who had their character sexually assaulted against their will by a DM who seemed dangerously casual about the whole thing. That player was young and awkward and afraid to stand up for herself, so she kept playing with that DM and the experience turned her off of D&D for decades. Obviously, that's the extreme example, but in those situations the smart thing to do is just leave the game... the DM isn't running a game that you want to be a part of, and forcing yourself to keep playing will only make you miserable.