Hyacidus

The Hyacidus is one of the functional flora added by Botania. The Hyacidus will, when supplied with Mana, inflict nearby mobs with the Poison status effect and bring them down to health point. Undead mobs, such as zombies and skeletons, are immune to poison, and are therefore unaffected.

Hyaciduss are not found naturally and must be planted by a player.

Trivia
The tooltip carries a reference to Pokemon on how Steel-type Pokemon are completely immune to any attack of the poison type.