Apparently this is a narrative he noodled with extensively during his lifetime but never completed to his own satisfaction, though he left behind a lot of manuscript pages; I think some version of it also appears in The Silmarillion. His son Christopher spent three decades piecing together a complete, if Frankensteinian, version of the story, which takes place in the First Age, a period I don’t know much about, except that it ended badly (I never finished The Silmarillion — there, I said it).
Christopher had this to say about the project: “It has seemed to me for a long time that there was a good case for presenting my father’s long version of the legend of The Children of Hurin as an independent work, between its own covers.”
Shyeah – a case FULL OF MONEY. Then again who knows, it’s probably really good.