Athelas

Athelas, also known as Kingsfoil, is a plant from Middle-earth in the Lord of the Rings continuum that has minor restorative properties which are much stronger when used by a king. It is the only known treatment for the Black Breath of the Nazgûl.

Due to the actions of PPC Bioengineering Inc, athelas has become a "secret ingredient" in many Real World medicines.

In Badfic
In badfic, athelas is the only medicinal herb in Middle-earth and is the go-to cure for everything. It may also be used for other purposes, such as cooking.