Mandrake Roots

Mandrake Roots are used by the Heron Guard and the Journeymen for their healing spells. Their healing ritual has its roots in the initiation rites of the Heron Guard, but what brings about immortality in a truly exceptional person merely restores the vitality of lesser men.

GURPS Myth
Their order teaches them to never use their own fatigue to do so in combat, for fear they'll exhaust themselves. These can be found for sale from time to time. Most of the expense reflects the difficulty of finding and procuring the root itself, which emits a ghastly shriek as it's unearthed that automatically slays any living digger. A variety of tricks - involving undead minions or Earth spells if available, cheap trained dogs if not - will circumvent this unhappy side effect.