champertor

from The Collaborative International Dictionary of English v.0.48

Champertor \Cham"per*tor\, n. [F. champarteur a divider of
   fields or field rent. See {Champerty}.] (Law)
   One guilty of champerty; one who purchases a suit, or the
   right of suing, and carries it on at his own expense, in
   order to obtain a share of the gain.
   [1913 Webster]

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

CHAMPERTOR, crim. law. One who makes pleas or suits, or causes them to be 
moved, either directly or indirectly, and sues them at his proper costs, 
upon condition of having a part of the gain.