baneful

adjective2

1. •
   Harmful, injurious.

   • This contempt of the understanding in early life has more baneful consequences than is commonly supposed […] — 1791
2. •
   (no longer used) Poisonous, deadly.

   • [S]ome fell serpent in his cave expects / The traveller's approach, batten'd vvith herbs / Of baneful juice to fury, forth he looks / Hideous, and lies coil'd all around his den. — 1791