The magical herb Hermes gives Odysseus is called moly. In Book 10 of the Odyssey, Hermes meets Odysseus before he reaches Circe's palace and provides the herb as protection against her drugs and enchantments.
Circe has transformed many of Odysseus's companions into pigs. Hermes warns Odysseus about her magic and explains that moly will prevent her potion from overpowering him. Odysseus can therefore draw his sword and force Circe to swear that she will not harm him, after which she restores his companions.
Homer describes moly as having a black root and a flower as white as milk. The text also says that the plant is difficult for mortals to pull from the ground but not for the immortal gods. Later writers added further stories about its origin.
Moly is not the same as nectar, ambrosia, or the lotus that causes forgetfulness elsewhere in the Odyssey. Scholars have proposed real plants such as snowdrop or wild garlic, but no identification is certain; the mythological name remains the canonical answer.