The Collaborative International Dictionary of English v.0.48:

Avenger \A*ven"ger\, n.
   1. One who avenges or vindicates; as, an avenger of blood.
      [1913 Webster]

   2. One who takes vengeance. [Obs.] --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

avenger
    n 1: someone who takes vengeance [syn: avenger, retaliator]