Who wrote...

[Gordis]

Originally Posted by mark12_30

As far as exactly who wrote it... in the Shadow of the Past, Gandalf only says : 'It is only two lines of a verse long known in Elven-lore".

He also says:

Out of the Black Years come the words that the Smiths of Eregion heard, and knew that they had been betrayed:
One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the Darkness bind them.

So these two lines were exactly what the Mirdain had heard.