Here is a plot that I have been developing:
Jonas, Herman, LOOK AWAY!
Around the time of troubles, the goddess mystra created a dark copy--a grim shadow--of Faerûn as it known today. Every person in the real world has a direct counterpart in this world, known as the shadow realms. These counterparts consist purely of the evil sides of every person, and this fact has made the shadow Realms a place of Havoc, Destruction and Slavery. There is only one catch: The shadow realms are somehow connected to the real realms, so that the only way to give birth to new life is if your counterpart in the real world does the same, creating a counterpart for the new creature in the real world. Because of the heavy lethality rate resulting from violence and havoc, Shar's realms are depopulating quicker and quicker.
Now, Shar has decided to try and merge the realms, transforming Faerûn into the dark world that is the Shadow Realms. The merging has already begun, and now the Shadow realms are partly occupying the plane of Shadow, the plane through any and all interplanar travel is performed. This means interplanar travel and teleportation (which also works through the shadow plane in my campaign) is blocked. This merging, sadly, needs a fantastic amount of magical energy. Shar has found a way to channel her own energy, but it requires a very strong magical device, for example a mythallar, the ancient devices of the netherils. Because of this, Shar has recruited the Shades from the City of Shade, who are her prime worshippers, to recover and reassemble the mythallars she needs. One is already located in the city of shade, but that is not enough. Information that the shades have been gathering these pieces, which are spread around Faerûn, have spread, and several kingdoms (Cormyr, Aglarond etc..) have recovered their own pieces and are guarding them with their life.
The lich Larloch, who was once a Netherese arcanist ruling his own flying city, as well as the Lord of the City of Shade, has been chosen to be the leaders of this grand operation. Also, to complete the trio, is the Death Knight Kargoth, from the Shadow Realms. The first step in the plan is to transport Kargoth to the real Realms. But because everyone has their counterparts in the real Realms, that counterpart must be destroyed before any transportation between the realms are possible.
Now, I have lot's more stuff about this plot in my head, but I'm too tired to write it all down right now. This is only the general outline of the campaign, and it's probably got ALOT of flaws. The players will be starting out in Trail's End in Damara, because that is close to Vaasa, the future base of operations of Larloch and his minions. Yes, he will reoccupy Castle Perilous.
Now I'm sick of writing. More later.