I'm not sure if hints are necessary or wise at this stage. We've covered quite a lot of possibilities, and any hint that's useful may very well end up giving the entire thing away.
I still have this thing in the back of my mind going "dark enchanted river running underground" over and over again, but I'm going to ignore that just now and guess:
The Mouth of Sauron
Dark as night: he
is a Black Numenoran, so check
Run far from light: more figurative that literal
Spell is upon me: did Sauron enchant or enslave him?
As you will now see: as you'll notice/observe
You'll beat me: Sauron was overthrown, and the mouth speaks for him
But I bring the woe: which the next bit
Your friend is now lost: Frodo, so check
But that was the cost: the cost of sending Shire-rats into Mordor
