Guardians and consorts of the first order of gods, the Sephirii were soldiers who enforced the original laws of the universe. During the Primus Bellum, the Mavromino turned against the Source and tried to end all creation, so the Sephirii were unleashed to kill him. In turn, the Mavromino created the Malachai to bring down the Sephirii. Most of the Sephirii flew into traps, but enough survived to declare war on the Malachai and would have destroyed them if not for the betrayal of their leader Jared. When the Primus Bellum finally ended in a truce, all the Sephirii and Malachai were supposed to be executed, but Jared's life was spared so he could suffer an eternity of slavery and torture as punishment for his crime. Because the universe requires balance, the survival of Jared allowed a Malachai to escape as well.
Due to their nature as guardians of the gods, they cannot harm any Source-borns, such as gods, unless they have declared intention to upset the order of the Source, such as Malachai.
Sephirii is the plural form of Sephiroth.