When the black breath blows
and death's shadow grows
and all lights pass,
come athelas! come athelas!
Life to the dying
In the king's hand lying!
Also known as kingsfoil or Asëa Aranion, athelas is a healing-herb found in middle earth. It has large, broad, green leaves that when crushed fill the room with most delightful scents, sometimes a cool breeze, sometimes orchids, sometimes a summer's day--scents so beautiful and pure they are beyond identification or desription. The herb is still effective when dried, although it loses some of its potency.
Once a very powerful, revered bit of medicine, in the later ages of middle earth, it fell out of use and was mostly used by midwives and village healers as an air purifier and not much more. Half-remembered snippits of the verse above left the belief it had once been more, but was lore lost to the ages.
Kingsfoil came back into its own under the hands of Aragorn, rightful king to the throne of Gondor. He brought Farmarir, Merry, and Eowyn back from near-death with an infusion of athelas in hot water and calling their names, calling them back from the brink.