Zappo said:
A gnomish industrial revolution resulting in a nuclear holocaust?
Well, actually that was threat easily averted. Now, 11th level magic and stuff does the trick. However, real problems came in guise of entropy loving pc:s, corruption and known worlds almost ending up in the Ravenloft.
Best way to create Apocalypse, is to make your players one's responsible for it. More involting story that way too. Know term 'accidental villains'?
Summoning of Old One's etc. is not that hard, all you need is some ignorant adventurer playing with wrong but 'oh so kewl' artifact.
And most important, remember to keep your goal of destruction in mind and not give in to whining characters (players are supposed to enjoy, well most of t he time anyway) or your need to protect the nice and wonderful world with all that 'stuff you don't really want to break'.
I am one of those (un)lucky players who manages play under destruction loving DM:s. Hehe, like it that way too.
FR has certain problem. It has so many threats, but none of them has truly come to pass. Evil in FR has high hype-factor, but in reality certain things remain pretty static. Makes it hard to find new more nasty thing to throw at there (not really problem, unless you have players who know every bit of FR lore there ever was).
And what comes to Crown of Horns or similar artifacts, WotC books describe artifacts really lame stat-wise these days. I really wish they wake up, and make evil and powerful artifacts not only, evil, but powerful too, and not just evil, but, powerfully including rule-wise cursed, but in a way, they seem like something somebody would want to start using.
Here is another 'god-politics' scenario:
Unthinkable happens. Shar manages to kill Selune, and steals much of her power. Greedily Shar feeds Shadow Weave more power, so she would be able to say Mystra good-bye for god, but doesn't take all things in considerance.
Terrible disharmony passes through whole world, Weave ends up mostly destroyed, magic goes crazy (Mystra along with it, if you want, or maybe Shar just kill her, before that can come to much).
Death of Weave kills Chosen of Mystra as their essesce along with their soul is ripped to oblivion.
However, Shadow Weave is not balanced enough creation: Terrible chaos in magic ensures, Dead magic, Wild magic everywere, weave users losing their powers, some dying, some going insane. Shadow weave casters are not doing well either, like said, that was not balanced enough to start with.
Plane of Shadow overlapping with Material plane, corruption and monsters created from poisonous wild magic and shadow-essence.
Shar is overjoyed. However, Shadow weave turns out to be unable to replace the Weave in all aspects. Gods' links to mortals fade. This includes Shar, who ends up trapped by her own follys.
Powerful, before unknown beings from Far Realms find their way though unhindered new ways, these wounds in primal nature of cosmos itself. Powerful, before unknown lords from shadow realms start their hunt for souls to rule or eat.
Power of gods doesn't stop demonic and devilic beings from entering, and some of them arrive to play their games and wage their otherworldy wars.
Maybe things are at this... or maybe more happens...
Some group of beings have power in this terrible cosmos, maybe those Shades, who have studied mystery of shadows for long and have more endurance for it's madness. Maybe there are seeirs out there, who knew this event would come to pass, and have made their arrengements.
Maybe some group manages to seal whole world from most of extraplanar elements, but unfortunely seal some of them in their side (wrong side), since it all happened 'so fast'.
Maybe you want to use psionic group of creatures who remain in powerful position.
Or maybe magic is mostly declared too dangerous (or maybe just too dead in some areas of the world), it's abardoned and barbaric hordes have rule of the day.
When you destroy a world, you have so much possibilities what to do with pieces left. And FR, well, they jumped there so many elements, just make your picks.
I just tried to combine few ideas given here.
Tired, Bored, had fun. Bye.