SEASON TEN AMD ELEVEN SUPERNATURAL SPOILERS!!!!!!!!'

Guys that woman isn't the Darkness.

Did she ever say she was the Darkness?

No

Did Dean ask if she was the Darkness or even mention to her the word Darkness?

No and no.

Think about it

"I don't know this Death and he doesn't know me"

Woah woah woah stop red flag Death seemed to know an awful lot about the Darkness didn't he? Right? He told Dean all about the Darkness!

And what really got me was when Cas was like "The Darkness is a woman" *mcsquinty confuzzle face*

Why would that surprise Castiel?

I mean he's possessed a female vessel before. Supernatural creatures tend to not really give a shiz about who their vessel is. The only explanation I can think of is Cas is like "wait I thought the Darkness was a giant black cloud that made zombies? Its possessing a vessel? That's unusual! Oh well I have other problems now!"

Finally if you rewatch the scene where the darkness emerges and pay attention to the top left corner you will also see a small shadowy winged figure emerging with the Darkness THAT IS THE WOMAN WITH THE MARK OF CAIN! THAT IS THE PERSON THAT PROTECTED AND SPOKE TO DEAN! THAT IS THE THING POSSESSING AMARA!

I'm sure hell has nothing to fear from the Daekness, it's just making zombies out of humans. As Crowley said "Hell and the Darkness, natural allies." No they need to be scared of little Amara she is the living embodiment of Chaos, she will devour your soul, that baby needs psalms, she is worse than the Darkness, she is why Lucifer screamed.