The Collaborative International Dictionary of English v.0.48:

nightmarish \nightmarish\ adj.
   Terrifying, as if in a nightmare[2]; resembling a
   nightmare[2].

   Syn: bloodcurdling, hair-raising.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nightmarish
    adj 1: extremely alarming [syn: bloodcurdling, hair-raising,
           nightmarish]