The Collaborative International Dictionary of English v.0.48:

Veneficial \Ven`e*fi"cial\, Veneficious \Ven`e*fi"cious\, a.
   Acting by poison; used in poisoning or in sorcery. [Obs.] "An
   old veneficious practice." --Sir T. Browne. --
   Ven`e*fi"cious*ly, adv. [Obs.]
   [1913 Webster]