I've been thinking about the mechanical specialties for the Spirits, and something is bothering me.
Here's what I've come up with so far:
Vital Strength of the Earth - Defending
Thunderspeaker - Dahan
Sharp Fangs Behind the Leaves - Beasts
Keeper of the Forbidden Wilds - Wilds
Lightning's Swift Strike - Fast Powers & Invader Destruction
Serpent Slumbering Beneath the Island - Other Spirits
Heart of the Wildfire - Blight
Ocean's Hungry Grasp - Oceans/Coasts
Bringer of Dreams and Nightmares - Fear
A Spread of Rampant Green - Growth/Presence
River Surges in Sunlight - Pushing
But Shadows Flicker Like Flame doesn't seem to have it's own unique specialty. I likes fear, but not as much as Bringer of Dreams and Nightmares does, and it likes Dahan but not as much as Thunderspeaker does (and Dreamer also likes Dahan)
On the other hand, I haven't actually had the chance to play the game much. Are my assesments accurate? What is it that makes Shadows Flicker Like Flames special?
"Do you not know that a man is not dead while his name is still spoken?"
- Terry Pratchett, Going Postal
Shadows Flicker Like Flame is a fear machine. All of its unique power cards generate fear. Higher levels of its innate power generate fear. It doesn't generate quite as much as Bringer of Dreams and Nightmares (especially in the late game, I've found), but it also doesn't have the limitation of not being able to damage/destroy the Invaders. Its innate power makes it pretty decent at dealing with explorers, pulling them out of lands before they build, but it isn't so strong against towns and cities. So, I would say that Shadows is largely about fear whereas Bringer of Dreams and Nightmares is exclusively about fear.
Comments on other spirits on your list:
Some additions to what grysqrl said:
I would say this is Shadow's mechanical specialty. The 0 range powers tend to have a high strength to cost ratio, and the ability to use these from across the board is huge.
I also think it's important to add that any spirit (with the exceptions of Bringer and Ocean) can eventually fill any role if you pick the right power cards. Don't get too caught up in sticking to your starting role.