Source : Webster's Revised Unabridged Dictionary (1913)

Baneful \Bane"ful\, a.
   Having poisonous qualities; deadly; destructive; injurious;
   noxious; pernicious. ``Baneful hemlock.'' --Garth. ``Baneful
   wrath.'' --Chapman. -- {Bane"ful*ly}, adv. --{Bane"ful*ness},
   n.

Source : WordNet®

banefully
     adv : in a noxiously baneful way; "this banefully poisoned
           climate" [syn: {perniciously}]