I like mine. It has been through a few iterations over the years, but the core concept has remained the same - rather than being people plucked out of the Prime Material, they're evil twins created by the horrible agony that was in the world around them.
The Shadowfell, being a reflective plane of the Prime, changes to mirror what happens in the "real world". Sometimes, when something incomprehensibly horrible takes place, the reflection manifests into an intelligent "twin" of something in the real world. It could be a being, a place or an object. Regardless of what the original was, the twin is evil and obsessed with something.
The Shadowfell treats these intelligent twins as invaders, and seals them up in a Misty Border that contains the infections. There are ways through the mist for everyone except the infection, but they are not easy.
I have some of the obligatory undead lords (Mummy Lord, Lich, Vampire, etc...), some insanely 'Haunted' locations, a few items of horrific interest, etc... but, also some that are the dark mirror of a hero of legend that was there when the tragic event took place. I've copied a couple high level PCs as twins.