The Collaborative International Dictionary of English v.0.48:

Baneful \Bane"ful\, a.
   Having poisonous qualities; deadly; destructive; injurious;
   noxious; pernicious. "Baneful hemlock." --Garth. "Baneful
   wrath." --Chapman.
   [1913 Webster] -- Bane"ful*ly, adv. --Bane"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

baneful
    adj 1: exceedingly harmful [syn: baneful, deadly,
           pernicious, pestilent]
    2: deadly or sinister; "the Florida eagles have a fierce baleful
       look" [syn: baleful, baneful]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "baneful":
   apocalyptic, appalling, atrocious, awful, bad, baleful, beastly,
   black, bodeful, boding, brutal, calamitous, cataclysmal,
   cataclysmic, catastrophic, consuming, consumptive, corroding,
   corrosive, corrupting, corruptive, counterproductive, damaging,
   dark, deadly, death-bringing, deathful, deathly, deleterious,
   demolishing, demolitionary, depredatory, desolating, destroying,
   destructive, detrimental, devastating, dire, direful,
   disadvantageous, disastrous, disserviceable, distressing, doomful,
   dreadful, dreary, evil, evil-starred, fatal, fateful, feral,
   foreboding, fratricidal, gloomy, grim, harmful, hideous,
   horrendous, horrible, horrid, horrific, horrifying, hurtful, ill,
   ill-boding, ill-fated, ill-omened, ill-starred, inauspicious,
   injurious, insalubrious, internecine, killing, lethal, lowering,
   malefic, malevolent, malign, malignant, menacing, mischievous,
   mortal, nihilist, nihilistic, noisome, noxious, of evil portent,
   ominous, pernicious, pestiferous, pestilent, pestilential,
   poisonous, portending, portentous, prejudicial, ravaging, rotten,
   ruining, ruinous, savage, scatheful, self-destructive, shocking,
   sinister, somber, subversionary, subversive, suicidal, terrible,
   threatening, toxic, tragic, unfavorable, unfortunate, unhealthy,
   unlucky, unpromising, unpropitious, unspeakable, untoward,
   unwholesome, vandalic, vandalish, vandalistic, venenate,
   veneniferous, venenous, venomous, vicious, virulent, wasteful,
   wasting, withering