May I guess Gurthang?
I'm as dark as night, - black sword, obviously
I run far from light, - I don't get this bit
spell is upon me - it's obviously enchanted by Eol, as it could talk
as you will now see. - filler stuff
You'll beat me, I know - it eventually broke
but I bring the woe: - while killing Turin
your friend is now lost, - obvious reference to Beleg
but that was the cost. - generic reference to Turin's doom