OK, rather than attempt an answer at this point, I'm going to post some speculation. Some of it may be bang on, some of it may be miles off. If anybody else uses this to determine the correct answer, well all's fair in love, war and riddles.
Some of this is in question form, but I don't expect an answer - it's merely intended to stimulate thought.
"I'm as dark as night"
Not
the night, just night. How dark is night anyway? Some nights can be fairly bright... And what is meant by "dark"? Mere absence of light, or "The Shadow"?
"I run far from light"
Run need not be the physical act of running, as I said a few posts back. What is meant by light? It need not be daylight... Again though, it's not
the light.
"Spell is upon me"
I read this as referring to something enchanted. Maybe not specifically enchanted (otherwise it would say "spell is put upon me"), but enchanted in nature.
"As you will now see"
See may refer to vision or to realisation.
"You'll beat me I know"
Legate has already confirmed that "I know" may be misleading here, and just exists to make the rhyme. Beat as in "defeat", or "overcome"?
"But I bring the woe"
A consequence of beating?
"Your friend is now lost"
Is this a reference to a specific incident?
"But that was the cost"
Again, is this this a consequence of beating?
And I'm afraid that my post had it's intended effect ..... on me.
So I will attempt an answer. Based on the above, I guess...
The Ring