The Arachnarok Spider uses a jolt of poison from its huge, envenomed fangs to kill more formidable opponents.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
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When fighting Monstrous Infantry, Monstrous Beasts, Monstrous Cavalry or Monsters; before rolling to hit, nominate one of the Arachnarok's attacks to be made with the Venom Surge, and roll it separately. This is a Poisoned Attack with the Multiple Wounds (D6) special rule.