Alright then, I'm going out on a limb and guessing that it's
Luthien's Hair Addressing The Two Remaining Silmarils In The Iron Crown
I'm as dark as night, - Luthien had dark hair
I run far from light, - her hair obviously ended up
very far from light
spell is upon me - she enchanted her hair
as you will now see. - and wasn't afraid to demonstrate the fact
You'll beat me, I know - she eventually died on account of her fairness when wearing the Silmaril burned her out
but I bring the woe: - which she did to Thingol
your friend is now lost, - the Silmaril they stole
but that was the cost. - the cost of beating her
This riddle is actually very very enjoyable.
