Collect 1 Essence of Tenacity from Rokmora in Neltharion's Lair, then slay Dar'grul the Underking.
Greetings, Archdruid. I hear that you seek the essence of tenacity.
There is none more unyielding and unbreakable than a creature of stone.
In Neltharion's Lair is a monster known as Rokmora. Slay him, and draw from him the essence that you need.