The Collaborative International Dictionary of English v.0.48:

Sadden \Sad"den\, v. t. [imp. & p. p. Saddened; p. pr. & vb.
   n. Saddening.]
   To make sad. Specifically:
   (a) To render heavy or cohesive. [Obs.]
       [1913 Webster]

             Marl is binding, and saddening of land is the great
             prejudice it doth to clay lands.     --Mortimer.
       [1913 Webster]
   (b) To make dull- or sad-colored, as cloth.
   (c) To make grave or serious; to make melancholy or
       sorrowful.
       [1913 Webster]

             Her gloomy presence saddens all the scene. --Pope.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sadden \Sad"den\, v. i.
   To become, or be made, sad. --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sadden
    v 1: make unhappy; "The news of her death saddened me" [ant:
         gladden, joy]
    2: come to feel sad [ant: gladden]