Athelas:

An herb brought from Númenor. In the rightful king's hands it acted as a cure for many ills. Aragorn used it in his attempt to heal Frodo of the wound caused by the Morgul-knife, though with little success. The dried leaves were more effective in the injuries sustained in the Mines of Moria however. Athelas was especially effective against the Black Shadow, as it affected Merry, Éowyn and Faramir. It did not grow wild in Middle-Earth, except in the areas where the Dúnedain had camped or lived.