[The Might]

I would say the Moon

For so long time I'm here - since the beginning of the First Age
though not from start of times. - of course
In greatest treachery
I at first took my part. - the first rising of the Moon marked the Return of the Noldor to Middle-earth

I saw how Darkness comes,
I saw how it departs.
I saw how it returns,
I saw the death of stars,
I saw corrupted hearts. - not hard to see everything from up there

Once They will come to me
to undo treachery - Melkor will destroy the Moon and the Sun before the Dagor Dagorath
at that day, it all ends
I'll die by my own hands. - not so sure about this