The Collaborative International Dictionary of English v.0.48:

Disseizor \Dis*sei"zor\, n. (Law)
   One who wrongfully disseizes, or puts another out of
   possession of a freehold. [Written also disseisor.]
   --Blackstone.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DISSEISOR, torts. One who puts another out of the possession of his lands
wrongfully.