I've heard of their lot. Men who can conjure Eidolons - summoners.
- —Kain Highwind
Summoners (召喚士, Shōkanshi?), also known as Callers, were a special group of humans in Final Fantasy IV that can summon Eidolons, or Call Beasts. Most of the Summoners lived in the town of Mist, in a valley northwest of Baron, although other summoners (specifically, an enemy Summoner) are seen in the game.
It is unspecified how the Summoners gained the ability to Summon. However, in a theme that repeats for many games afterwards, the "law of summons" is put forth as a code of conduct between Summoner and Monster. This code is written as "test the caller before you answer the call" - in other words, only once the Summoner defeats the Eidolon in combat can they summon it. Thus it could be inferred that Rydia faced and defeated all of her summons prior to rejoining the party, although it is also possible she merely befriended them and they waived the law.
Most other summons in the game, such as Asura, Leviathan, Odin, and Bahamut, must be faced and defeated before Rydia can summon them. Despite the great power the monsters possess, Summoners are not without risk when summoning - if their Eidolon is slain in battle, the Summoner dies as well. It is implied in Final Fantasy IV: The After Years that the Summoner race and the Maenads were created in the same way, although at different times in the Final Fantasy IV world's history. Leviathan and Asura even remark how Cuore looks like Rydia when she was a child.
Summoners also apparently can wield magic quite efficiently, and Rydia as a child can cast White and Black Magic. According to Rosa Joanna Farrell, Summoners are naturally attuned to Black Magic, and when Rydia is taken to the Land of Summons and returns, her abilities at both Summoning and Black Magic have increased, but her ability to cast White Magic was lost in the process.
The Summoners were wiped out by Cagnazzo under the guise of the King of Baron, possibly because he feared that the spirit of the dead king could be summoned as Odin. This was carried out unknowingly by Cecil Harvey and Kain Highwind, when a ring they were told to deliver to Mist spawned Bombs that razed the village. Most of the Summoners were killed, as were the women and children so the townspeople could not reproduce. Rydia survived the burning, however, and went on to become a highly skilled Summoner.
In his battle with Cecil, it is also shown that Golbez is capable of summoning a Shadow Dragon to attack them, until it is defeated by Rydia's Mist Dragon. However, this is the only time Golbez is seen Summoning, and he is not of the Summoner race.