"Evokers are initiates of summoning magic, able to randomly draw either the light or dark power of summoned beasts."
The Evoker(幻術師,Genjutsushi?, lit. "Illusionist"), also known as Conjurer, is a Job class in Final Fantasy III. It is gained from the Water Crystal. Evokers possess the ability to summon powerful beings into battle. Each possesses a direct attack (Black) and an indirect effect (White), which is triggered randomly at the time of summoning. For example, the spell Catastro summons Odin. Odin can use Slash (direct attack), or Protective Light (indirect effect).
The Conjurer is the first Job you get that can summon monsters. However, there is a "White" and "Black" side to each summon. The White side generally either benefits the party by buffing all characters or debuffs all monsters with 100% success rate. The Black side generally attacks a single monster ignoring Magic Defence. There is a 50 - 50 chance of getting either effect. Once the Summoner Job is available, it is best advised to switch to that one.