How to destroy the Realms?
First, something should happen to Elminster, something that causes him to go insane - permanently. Maybe Manshoon finally defeats his hated enemy, but instead of killing him, he renders him permanently insane. This is not just cruelty on his part, it is all part of the plan. With Elminster gone mad, the other Chosen of Mystra must rush about the Realms, trying to locate the Old Mage and repair any damage he has done - ans since he's the second most powerful Wizard in the Realms, that's a LOT of damage.
But couldn't Mystra just cut off his access to the Weave? you ask.
Why, yes, she could.
But what if Shar has invested Elminster with the ability to tap into the Shadow Weave?
So. You have Elminster running amok, and the Chosen trying to find him. What happens next?
Glorious Chaos. The Zhentarim prepare to invade the Dalelands, and then push their way south. But with what army? Why, with the army from their secret staging ground of Vaasa.
Vaasa?, you ask. Oh yes, Vaasa. And they have dragons with their army. The same dragons that have been flocking to the ruins of Castle Perilous, the result of being summoned there by a very evil, very powerful being - the Banelich.
So. Elminster insane. The Chosen running around like headless chickens. An invading army approaching the Dalelands, accompanied by a flight of dragons.
Eventually, the Zhents will control all lands north of the Dragonmere and the Pirate Isles, and west of the Dragon Reach. Thay will have Aglarond, and will begin looking northwards towards Thesk, The Great Dale, and possibly even Narfell.
Some of the Chosen fall - either battling the Zhentarim, the Dragons, or even Elminster himself. At the least, Dove, Storm, and the Symbul all perish in the chaos. If your especially sadistic, you can have the Symbul killed while trying to fight Elminster.
Strangely enough, Khelben is strangely absent from the battles, though Laeral immediately goes to the aid of her sisters.
More could be said, and done, but I'll leave that to you.