So in my campaign, the BBEG is Asmodeus. The setting for my campaign is Faerûn. The PC's are about to kill Asmodeus's right hand man. The moment after his death, Asmodeus will come from his corpse and rip apart Faerûn. The continent will become black ooze/tar, and be a land for evil creatures. The PC's and most people of Faerûn will have a chance to flee before Faerûn falls. Hopefully the PCs will get on ship, after wandering aimlessly, they will land on Wildemount. I thought it would be really cool, because most of the world of Toril is unknown. The PCs would be entroduced to new politics, gods, and leaders. 

Now I know that Exandria and Toril are completely different worlds. But I just got the Explorers Guide to Wildemount and I really want to use it in my campaign. So all I'm asking is:

1. Is this a good idea?
2. Any critics?
3. If it is not a good idea, why should I not do it? 

Well its kind of the definition of a railroad. You know what’s going to happen before you even start the session. Do you even need the players there for it?

What if you just told them, out of character, you just got the wildemount book and want to try it out and move their characters to the new setting. Get them on board with it and then figure out an option together. Maybe they cast the blue veil spell from Tasha’s, maybe they like like the idea of burning down the realms on the way out the door. Maybe they like those character generation options on wildemount and want to start a new campaign, so you all figure out a way to speed up the current one to the end so you can start a whole new one.

But your idea of forcing it on them could really lead to some annoyed players. 

Totally didn't think of this! I thought it could be a good plot thing.

I'd agree with Xalthu on this one.  A surprise world-ending event that the party has no way of stopping could be a bit of a downer, especially if they "did everything right", so to speak.  If your peeps are interested in trying Wildemount there's a ton of ways to get them there without destroying Faerûn, which may be more attractive to them if they've built connections with the people/places there.  

Also, in character -- why wouldn't someone from among the gods try to stop Asmodeus from turning an entire continent to black tar?

Also, how could "most people of Faerun" have a chance to flee? Is it going to take months or years for the world to turn to black tar? And if so, why wouldn't the PCs want to use that time to stop the "tarring" of the land? And if Asmodeus is the BBEG, how does he come back into the story once in Wildemount? It seems like you are tarring over the original world, and then Asmodeus will be out of the story and some new villain will be present.

I agree with Xalthu and Beyodrae. This is a terrible thing to do to your players arbitrarily and without warning. Maybe some of them don't like Wildemount, you know? Or prefer Faerun, anyway. Maybe some of them have ties here, and would like to keep those ties. Maybe they have family. Maybe one planned to retire in Waterdeep. Turning the entire land into tar and forever wrecking it, doesn't sound like fun at all.

As a DM, you have a responsibility to your players not to just jump around from one campaign setting to another on a whim because you think the new setting is "cool." Sure, it's cool. Set your next campaign there. But as a DM, you have the obligation to finish what you started, and not hop around from one cool thing to another.

After all, imagine you came here saying that you and your table were playing Out of the Abyss, and you are in the middle of it around level 6-7. But you just bought Rime of the Frostmaiden and think it's a better adventure... how do you get the existing part from OOA to ROTF? The answer is, you don't... you have some stick-to-it-iveness and finish OOA first, and THEN do Rime. Or you speak to your players and tell them, look, you have read all the way through both OOA and ROTF, and ROTF is clearly a better adventure you could all have more fun with. Would they mind switching? Then ask, "OK with these characters or new ones?" (I'd bet any money they'll all say "new ones!" because making up characters is super fun.) But you don't just get a bunch of people all into playing OOA, and then as a DM, decide you're bored of OOA and now it is time to switch over to Rime. That's rude -- and selfish.

If you really want to play in Exandria, I think your best bet is to talk to the players and suggest, putting this current campaign on hold, making up a new party, and doing Wildemount right, keeping its internal mythology (which is different from FR) and so forth. Maybe you guys will love it and never go back to the old party. Maybe you'll hate it and want to go right back to Faerun. This allows you to play the game you want, and your players want, without hosing the existing campaign just because you got a new book that was cool.

I agree with what's been said so far, and I've think s better alternative would be to reward your players' victory over Asmodeus with the opportunity to explore a whole new continent's worth of adventure, rather than punishing their success and making them start over in a new land. 

Maybe after Asmo is defeated they hear tell that an expedition is heading out to the far far west and they're looking for adventurers to protect the expedition. Maybe even when they defeat Asmo the party cleric has a strange sense that he's still not gone from the world and they sense that part of him survives in a far off, unknown land. 

That way you get your players excited and invested in the opportunity to explore the content you're also excited about.

Okay so after reading all of this, here is my new idea:

A strange black tar like substance is slowly spreading in the places of darkness from Faerûn. The scientists of Faerûn have studied and say that it is spreading very fast, they have two weeks. At the end of the two weeks a worldwide evacuation is called. The PCs are asked to find out what it is, and try and stop it. There is chance they will fail and there is a chance they will win. 

so they have a chance to win or they don't. The thing that will stop the tar will be an artifact i created. I have a bunch of lore set up things like that. Is that better?

I am a fairly new DM, and to be honest, I was pretty crushed to hear that I was railroading. (Note: I have not done this plot hook yet) I've been doing my best not too, but am I happy to learn from the experience!

Holy wow. I think the most important thing is that your players (a) aren’t invested in Faerun and (b) are invested in Wildemount. Having enjoyed a number of adventurer’s league games, and never really having gotten into Critical Role, I know I wouldn’t fit either description. As a result, I’d be upset to see Faerun get apocalypsed and the campaign move to a world I don’t care for, with no warning. But if your group likes CR and doesn’t care about FR continuity, it could be cool.

I do like the revised idea, it sounds like a fun campaign. But where (if anywhere) does Wildemount fit in? For my two cents, I agree as I often do with BioWizard: finish out this campaign for your players, then play the next one in Wildemount. That would be cool.

I am a fairly new DM, and to be honest, I was pretty crushed to hear that I was railroading. 

The issue is not railroading. The issue is the fickleness of hopping from one setting to another completely different setting, arbitrarily, just because you think it's cool and want to try it out, without getting any input from your players.

This is not to say that the players get to control the setting -- that is the job of the DM. But also the job of the DM is trying to put in front of the players a setting you think they will like. As I said earlier, maybe someone doesn't like Wildemount? For example, I do not. As much as I dislike FR, and recognize that Mercer's world is more coherent than FR (which is utterly IN-coherent as a world concept), I don't actually like his world any better. A coherent concept that I dislike is not superior to an incoherent concept that I neither like nor dislike. So for my money, as a player, although I love neither, FR > Wildemount.

Now, that is MY opinion, but if you pulled this as my DM arbitrarily you'd probably be getting a sad email from me saying I am pulling out of the campaign, because you've switched us from a setting I don't love but can tolerate, into a setting I actively dislike.

Now, maybe your players would like WM way better than FR. If so, great. But you should talk to them first. This is common courtesy. D&D is a cooperative venture, and the DM may be the one who interprets the rules and designs the setting, but the players all have skin in the game too, and swapping settings on them mid-game is not fair. So the most important thing here is to get your players' input.

Quote from BioWizard >>

I am a fairly new DM, and to be honest, I was pretty crushed to hear that I was railroading. 

The issue is not railroading. The issue is the fickleness of hopping from one setting to another completely different setting, arbitrarily, just because you think it's cool and want to try it out, without getting any input from your players.

This is not to say that the players get to control the setting -- that is the job of the DM. But also the job of the DM is trying to put in front of the players a setting you think they will like. As I said earlier, maybe someone doesn't like Wildemount? For example, I do not. As much as I dislike FR, and recognize that Mercer's world is more coherent than FR (which is utterly IN-coherent as a world concept), I don't actually like his world any better. A coherent concept that I dislike is not superior to an incoherent concept that I neither like nor dislike. So for my money, as a player, although I love neither, FR > Wildemount.

Now, that is MY opinion, but if you pulled this as my DM arbitrarily you'd probably be getting a sad email from me saying I am pulling out of the campaign, because you've switched us from a setting I don't love but can tolerate, into a setting I actively dislike.

Now, maybe your players would like WM way better than FR. If so, great. But you should talk to them first. This is common courtesy. D&D is a cooperative venture, and the DM may be the one who interprets the rules and designs the setting, but the players all have skin in the game too, and swapping settings on them mid-game is not fair. So the most important thing here is to get your players' input.

 I have talked to my players and they have all agreed that they would like to explore Faerûn, then move on to Wildemount, which is perfect! 

I was planning on giving them quite awhile to explore Faerûn, get to know various NPCs, and other things like that. The point of Faerûn getting destroyed is to give the PCs the sheer amount of feelings of loss and grief/uncertainty. To show that the BBEG they are up against is really no joke.

As for my BBEG, Asmodeus, and why the other gods would not interfere with Asmodeus destroying Faerûn. It would take a really long time explaining lol. This is a homebrew (kinda) campaign, so I chose my own gods and things like that. Its really hard to explain. 

tl dr: So is it a good idea, since my players are wanting to do that? Mostly asking is the storyline good?

If your players want to do it, then I think you're all set.

I'd approach it by having the blight start spreading in Faerun, and have them pushed towards Wildemount, EG "there is a sorceror there who knows of these things, take this letter to him with all haste, and pray you do not return too late!"

Thus the players can go off to Wildemount, and have some adventures there. Depending on how much they divert from the quest to save Faerun, the blight may have spread different amounts. If they spend their time exploring Wildemount and ignoring the goal, they might return from a dungeon to find the ragtag stragglers of an exodus arriving in port, telling of how Faerun has fallen. oops, should have found that relic sooner! It gives you, as the DM, complete control over how bad Faerun is when they get back. You might have the sorceror use a teleportation circle to send them back for something from a library in Faerun, and they arrive to see the blight is really affecting the land, as a reminder of its progress towards becoming a big glob of tar.

I agree that the original concept is unfair to the players but the core of it has a lot of merit.  I would heavily foreshadow the apocalypse with some hard wired start date and, instead of having it start when the players slay Lieutenant Boss, have him scream dying words that they were too late, the great melt-down will start soon anyway.  Then give them SEVERAL solution hooks each of which happen to take them to Wildemount.  Maybe build Wildemount into Lieutenant Boss's backstory.  Once in Wildemount, keep entangling them in local affairs as the clock ticks away back home.