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]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "sadden":
   affect, aggrieve, beat down, cast down, damp, dampen,
   dampen the spirits, darken, dash, deject, depress, discourage,
   dishearten, dispirit, distress, grieve, knock down, lower,
   lower the spirits, melt, melt the heart, move, oppress, press,
   press down, reach, sink, soften, sorrow, touch, weigh down,
   weigh heavy upon, weigh upon