[The Might]

Well...I have had no better idea till now
Maybe it is the secret tunnel in the Lonely Mountain

I'm as dark as night, - and it is
I run far from light, - beneath the Lonely Mountain
spell is upon me - the spell that it is only accesible when the it is opened with the key while the last sun ray on Durin's Day shines on it
as you will now see. - as was seen by the Dwarves and Bilbo
You'll beat me, I know - the Dwarves swore to return to Erebor one day
but I bring the woe: - but...
your friend is now lost, - The Dwarves thought that Bilbo was lost at first after hearing strange sounds
but that was the cost. - for entering Erebor and stealing something from Smaug's treasure

I really can't think of anything else, and since you said the net was closing tight when talking about the Paths of the Dead I thought it must be some tunnel
Since Moria already was mentioned I guessed I should at least give this a try...