Insight
Adventurer
Bwaaarrr...I like it. I like how that lends itself to the idea that monsters are "twisted" due to exposure to this strange force, this Shadow. It also leaves open the question of what, exactly, the Shadow is. An alien energy from another universe? Some kind of bizarre bioweapon gone amok? A magical curse? The subconscious horrors of the collective human mind made manifest?
It gives a much larger depth to the struggles and battle than just a 'monster du jour' would be. Every encounter we have is a chance to learn more.
Yes!
And, I'd like to point out that this idea is still in the formative stages. If you have additional ideas, please don't be shy in tossing them in the ring. I want to make this game as much about the players as whatever I manage to come up with.