[The Might]

Maybe Ungoliant

I'm as dark as night, - which she was, also in earlier versions an incarnated form of the Darkness
I run far from light, - she fed on light, still she hid herself from it
spell is upon me - maybe because she had followed Melkor in the past
as you will now see. - and was summoned by him later
You'll beat me, I know - perhaps talking about the balrogs
but I bring the woe: - destroyed the trees
your friend is now lost, - Finwe killed at Formenos
but that was the cost. - To get the Silmarils