The Fall of Arthur? Tolkien's long (almost 1,000 lines), unfinished Arthurian poem? The one with clear affinities to the fourteenth century Alliterative Morte Arthur? The one praised by E. V. Gordon, and R. W. Chambers, and C. S. Lewis? That Fall of Arthur?
Dare we hope? I remember Rayner Unwin, when I got to meet with him in 1985, telling me about this as one of the forthcoming projects already in the works, but which wdn't be coming out until some more pressing projects (like the HISTORY OF MIDDLE-EARTH series, whose third volume I'd just picked up that same day).*
This is one of the unpublished Tolkien works I've most been wanting to see in print. We'll see if the rumors are just rumors, or if it's really on the way. I'd say the whole thing is wishful thinking (I know the PULP CTHULHU rulebook still gets listed on amazon and similar sites, despite never having been published), but the precision of May 13th 2013 as its release date hints that perhaps it's more than that.
*I wound up giving that volume to Owen Barfield, who was interested to hear that Lewis had critiqued some of Tolkien's poetry and was curious to see those comments for himself