[The Might]

I'm thinking of Turin

I'm as dark as night, - In reference to his sword and perhaps secrecy
I run far from light, - He did run through the tunnel of the Noldor twice
spell is upon me - doomed by Melkor
as you will now see. - as seen after he slays Glaurung
You'll beat me, I know - talking to his sword
but I bring the woe: - not sure which one is meant here, Turin was responsible for many
your friend is now lost, - Beleg Cuthalion, the former owner
but that was the cost. - perhaps the cost for Turin to receive the sword