[Thenamir]

I'm barging in here, with not a single post on this thread heretofore (and kudos to Legate for not only the composition, but the artful replies to the guesses so far) but something jogged my mind in the last two lines of this one. My guess is the Ring itself, speaking to Gollum, perhaps at the Crack of Doom, but I can't make it all fit. Here's what I have:

I'm as dark as night, - not sure about this one, perhaps though it was shiny gold, it was dark in purpose, not in color.
I run far from light, - the darkness of the Barad-Dur and perhaps also of Gollum's cave?
Spell is upon me - the spell of the Dark Lord at the Ring's forging, perhaps the inscription? (hmmm...spell...inscription...could this be a horrible pun?)
As you will now see. - another I'm not sure about, perhaps the fact that the inscription appears when heated, as it undoubtedly did when cast into the chasm, depicted in the movie but not explicitly mentioned in the book.
You'll beat me, I know - Gollum was the cause of the eventual destruction of the Ring
But I bring the woe: - fairly obvious for all who carried or desired the Ring
Your friend is now lost, - the murder of Deagol
But that was the cost. - Smeagol could not obtain the Ring otherwise

Not as creative as some of the guesses so far, but I had to get rid of the tickle in my mind. Knowing the creativity of the riddles so far, it can't be that obvious...or can it? Great thread, keep it up!

EDIT: I thought I'd read the progress of the riddle carefully, but I see that someone already guessed the Ring. My apologies for the side trip. But I contend, based on the explanations above, that the Ring fits fairly well, if not exactly.