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Synonyms for Baneful (same or very similar meaning)

WordNet sense 1 (exceedingly harmful):
deadly, pernicious, pestilent

WordNet sense 2 (injurious to physical or mental health):
harmful, noxious

WordNet sense 3 (deadly or sinister):

WordNet sense 4 (harmful or evil in intent or effect):
maleficent, maleficent

From the ODE community, based on WordNetadd/edit