I believe I was wrong in thinking it was linked to the Dagor Dagorath so long
I would say it's Ungoliant
For so long time I'm here - Long time in Arda
though not from start of times. - She came from the Outside Darkness at a certain time
In greatest treachery
I at first took my part. - Served Melkor in the beginning
I saw how Darkness comes, - The coming of Melkor to ask her for help
I saw how it departs. - Towards the Trees
I saw how it returns, - His return to Valinor
I saw the death of stars, - Maybe the death of the Trees
I saw corrupted hearts. - The Elves or Melkor...
Once They will come to me - the balrogs
to undo treachery - her treachery to Melkor; trying to take the Silmarils from him
at that day, it all ends - afterwards she passes South and doesn't trouble anyone anymore (or at least we know this not)
I'll die by my own hands. - she devoured herself