[The Might]

Great riddle I must admit...I have been thinking about a possible answer the last few days, but nothing really good came to me until know

I personally think of the Dark Door, the entrance ot the Paths of the Dead

I'm as dark as night, - obviously since it is called the dark door
I run far from light, - it is found in a dark glen
spell is upon me - the prophecy made by an old man sitting there to Baldor of Rohan ([font=Verdana][size=-1]"The way is shut. It was made by those who are Dead, and the Dead keep it, until the time comes.")[/size][/font]
as you will now see. - now means that time had come, Aragorn had arrived to fulfill the prophecy
You'll beat me, I know - just as above, it was time for Aragorn to pass the door and enter the Paths of the Dead
but I bring the woe: - I explain this below
your friend is now lost,- talking about Halabarad of the Dunedain, he himself says as he reaches the door [font=Verdana][size=-1]"This is an evil door, and my death lies beyond it. I will dare to pass it nonetheless..."[/size][/font]
but that was the cost. - the cost of Aragorn passing

As you said, I think now I feel I found the right answer.