[Beregond]

Well, I'm going to try again, and I fear I'm way off course now, and I'm not sure I've retained the two crucial points I hit before, but it can't hurt to try..and you haven't said it's a person....


Beleriand?

For so long time I'm here - yes, very long
though not from start of times. - not quite
In greatest treachery
I at first took my part. - Melkor fled to Beleriand and built Utumno there (unless it was further north?)

I saw how Darkness comes, - The first coming of Morgoth
I saw how it departs. - Morgoth defeated
I saw how it returns, - Morgoth come again to Angband
I saw the death of stars, - The Silmarils thrown into the sea and into a fiery chasm
I saw corrupted hearts. - Many many corrupted hearts it would have seen.

Once They will come to me -
to undo treachery - the Valar came to Beleriand, defeated Morgoth, and pardoned the Noldor
at that day, it all ends -
I'll die by my own hands. - Beleriand was destroyed and sunk into the sea......