These heretic mages have unlocked the secrets of the forbidden magic of Vana'diel by creating a pact with the Avatar known as Carbuncle. Through his suggestion, these mages have the choice to delve further into the craft and create pacts with many other Avatars of Vana'diel after defeating them in battle to prove their worth. If accepted, Summoners (SMN) are able to fight alongside them in order to bring peace to Vana'diel.
The Summoner job is available after completing the quest I Can Hear a Rainbow. You must level Summoner to 75 to unlock this achievement. For more information on the Summoner, click here.