The Collaborative International Dictionary of English v.0.48:

Tenebrific \Ten`e*brif"ic\, a. [L. tenebrae darkness + facere to
   make.]
   Rendering dark or gloomy; tenebrous; gloomy.
   [1913 Webster]

         It lightens, it brightens,
         The tenebrific scene.                    --Burns.
   [1913 Webster]

         Where light
         Lay fitful in a tenebrific time.         --R. Browning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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