Judging from the codes given for Nightmare decks on the FFG page for the LotR LCG, I'd guess it's for The Road Darkens.
Numbers 1-3 are the Core scenarios, 4-9 the Shadows of Mirkwood cycle, 10 Khazad-Dum, 11-16 the Dwarrowdelf cycle, 17-19 the two Hobbit and first LotR Saga expansions, 21 Heirs of Numenor, 22-27 Against the Shadow, and 28 the Voice of Isengard. (The page doesn't list 18 or 20, but it seems obvious that 18 must be On the Doorstep.)
[Edit on 24 May 2016: the FFG page (referenced below) now has a complete list of Nightmare Decks from 1 to 34, and the two guesses above for 18 and 20 are confirmed, and 29-34 are (as you'd expect) for the Ring-Maker cycle.]
Last edited Tue May 24, 2016 4:35 pm (Total Number of Edits: 1)