I have returned!
And I have a strong hunch the answer is Túrin.
Here's my thoughts, mostly based on the first four lines.
To first of you, I'm just a tale, - No idea who the first person is
told by the skies, as they unveil - In the oldest tradition, The Lost Tales, Menelvagor is the enemy of the exiled Melkor, protecting Arda from his return. In later developments of the story, the stars came to represent none other than Túrin, who was fated to return from the dead and face Melkor in the Last Battle.
mysteries hidden long ago. - This tradition can be seen in the words of the Silmarillion, where Menelvagor 'forebodes the Last Battle that shall be at the end of days.'
I don't have many ideas for the other lines and it's late so I can't come up with much.
