They are reminiscent of the Four Fiends from the original Final Fantasy, as monstrous henchmen of the main antagonist related to the crystals. Such groups of creatures were common in the Nintendo-era Final Fantasy games, and also include the Archfiends from Final Fantasy IV, Demons of the Rift from Final Fantasy V, and Guardians of Terra from Final Fantasy IX.
After the death of Xande the Cloud of Darkness enters the World of Darkness to complete her plans of destroying both worlds with the powers of The Void. When the Warriors of the Light come to stop it Cloud of Darkness is too powerful for them to defeat. The Warriors of the Light search for the Warriors of the Dark to weaken it and find the Dark Crystal. The Cloud of Darkness orders its guardians to guard the Dark Crystals and kill the Warriors. Each guardian appears when the Warriors of the Light touches their Crystal:
Each time one is defeated by the Warriors of the Light, one of the Warriors of the Dark appears to help the Warriors of the Light by giving them important information and weaken the Cloud of Darkness. Eventually all of the guardians and the Cloud of Darkness are defeated by the combined power of both Warriors of the Light and Dark.
The Guardians appear as bosses on the 12th floor of the Depths.
Different variants of the Guardians appear as bosses in the World of Darkness raid, the Angra Mainyu, the Five-headed Dragon, and Cerberus. The Angra Mainyu is based on the design of Ahriman, the Five-Headed Dragon is loosely based on the Two-Headed Dragon, and Cerberus is based on the boss of the same name.
In addition, the Two-Headed Dragon appears as a normal enemy in the first set of enemies in the raid.
The Two-Headed Dragon appears twice in trial battles in the Chamber of the Fallen and Chamber of the Indignant. It yields the Grand Helm and Grand Helm+ respectively upon defeat.
In the global version, Echidna appears as a boss on her own in the Lure of Echidna trial, as well as in the second stage of the three-part Torturous Trio trial. Defeating Echidna in her own trial will yield the Assasin's Vest, and if she is defeated by an esper attack, she will also give the Echidna's Kiss materia, which boosts the user's MAG by 30% of their base.