The Collaborative International Dictionary of English v.0.48:

Tenebrous \Ten"e*brous\, a. [L. tenebrosus, fr. tenebrae
   darkness: cf. F. t['e]n['e]breux.]
   Dark; gloomy; dusky; tenebrious. -- Ten"e*brous*ness, n.
   [1913 Webster]

         The most dark, tenebrous night.          --J. Hall
                                                  (1565).
   [1913 Webster]

         The towering and tenebrous boughts of the cypress.
                                                  --Longfellow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tenebrous
    adj 1: dark and gloomy; "a tenebrous cave" [syn: tenebrous,
           tenebrific, tenebrious]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "tenebrous":
   ambiguous, amphibological, beamless, black, black as night,
   caliginous, dark, dark as night, dark as pitch, darkling, darksome,
   dim, dusk, dusky, ebon, ebony, eclipsed, equivocal, gloomy, murky,
   night-black, night-clad, night-cloaked, night-dark,
   night-enshrouded, night-filled, night-mantled, night-veiled,
   obfuscated, obscure, obscured, occulted, pitch-black, pitch-dark,
   pitchy, rayless, sibylline, starless, sunless, tenebrious,
   tenebrose, uncertain, unclear, unilluminated, unintelligible,
   unlighted, unlit, vague