Athelas is a fictional healing herb from J.R.R. Tolkien's Middle-earth, also known as Kingsfoil or Asëa Aranion.
It was first brought to Middle-earth by Númenoreans, but by the end of the Third Age the knowledge of its healing properties was lost among all but the Rangers of the North. According to folklore in Gondor, it is especially powerful in the hands of the king, perhaps because of the Elvish heritage of the royal house. It was used by Aragorn, secretly entering Minas Tirith upon his return to Gondor to heal those touched by the black breath, an act that enhanced his reputation and strengthed his claim to the crown.
In the Lord of the Rings movie, Arwen uses athelas gathered by Aragorn and Sam to heal Frodo. In the book, at the same point in the story, Aragorn is the one who prepares and uses it.
Athelas was also used by Huan and Lúthien to heal wounded Beren in the early Lay of Leithian. It contradicts the information from The Lord of the Rings of it being brought to Middle_earth by Númenoreans, so it was either changed by Tolkien in later versions of his legendarium, or athelas grew in Beleriand before it was destroyed, and then brought back to Middle_earth by Númenoreans in the Second Age.
Athelas could be used to ease pain, but its most important property was its power to counteract the effects of the Black Breath exuded by the Nazgul.
Athelas was brought to Middle-earth from Numenor and could be found growing in places where the Men of Westernesse used to dwell or camp, including the area around Weathertop and the woods of Lossarnach in Gondor.
Frodo felt the pain and cold lessen, but the athelas could not cure him since there was still a sliver of the Morgul-knife in his body that had to be removed by Elrond.