In the Dark Tower series, when is the Horn of Eld first mentioned? I'm trying to understand how far into writing the series Stephen King planned things.
The gunslinger waited for the time of the drawing and dreamed his long dreams of the Dark Tower, to which he would some day come at dusk and approach, winding his horn, to do some unimaginable final battle.
There is another probable reference to the Horn of Eld in the third book:
He cried out but his cry was lost in the golden blast of some tremendous horn. It came from the top of the Tower, and seemed to fill the world. As that note of warning held and drew out over the field where he stood, blackness welled from the windows which girdled the Tower.
—The Waste Lands
The first reference to the Horn as belonging to Eld is, I think, in Wolves of the Calla:
His shirt is soaked crimson to his skin. One side of his face has been drowned in blood; the eye on that side bulges sightlessly on his cheek. Yet he still has Roland's horn, the one which was blown by Arthur Eld, or so the stories did say.
—Wolves of the Calla
So it would seem that King had some idea of the Horn of Eld as early as the first book.
This is not so surprising, considering the strong mythological association between Roland and his horn.
Considering the novel is heavily influenced by Robert Browning's 1885 poem "Childe Roland to the Dark Tower Came", the final couplet of which is
Dauntless the slug-horn to my lips I set, And blew "Childe Roland to the Dark Tower came."
I'm fairly sure that King has some notion of the place of the Horn of Eld right from the beginning.
Interestingly, Michael Moorcock, in "Stormbringer", also had his multiversal traveller Elric of Melnibone recover the Horn of Roland from a tower and blow it.