The Custodians (カストディアン Kasutodian?), also known as the Anunnaki (アヌンナキ Anun'naki?), are a race of highly advanced space-faring extraterrestrial beings who created and ruled over prehistoric humanity, the Lulu Amelu, and were responsible for the Curse of Balal.
The Anunnaki are a group of deities appearing in Sumerian, Akkadian, Assyrian and Babylonian mythologies. Descriptions of their number and role vary between tales, from gods of the heavens who created and decree the fates of humanity to judges residing in the underworld.
In the ancient past, before humanity existed, the Custodians arrived on Earth using interstellar cruise ships such as the Frontier. They attempted to create life with their first prototype, Adam Weishaupt, but due to being created "perfect" with no potential for growth or evolution he was cast aside and forgotten. Later on, they attempted to create life again using both qualities they liked and disliked, resulting in humanity, which they accepted. They appear to have acted as deities to pre-historic humanity, specifically Sumerian and Babylonian civilizations, and either through gifting it or some other means their highly advanced technology and the means to replicate it came into the hands of humanity.
In S1, Finé tells the tale of how she, a priestess from the prehistoric era, fell in love with 'God', and built the Tower of Babel to confess her love to them. However, God would not allow a mere human to stand on the same level as them, and destroyed the tower, before activating the Curse of Balal via the moon to scramble humanity's languages. This robbed humanity of the unified language as well as mutual understanding. Finé, enraged, began her millennia-long journey to find a way to remove the Curse of Balal and then become the ruler of the Earth.
In XV, it is revealed that the 'God' of Earth is Shem-Ha Mephorash, and the one who activated the Curse of Balal was Enki, a custodian whom Finé had 'snuggled close to' prior, to protect humanity from Shem-Ha reincarnating through the unified language. Enki, heavily wounded from his battle with Shem-Ha, died before he could explain everything to Finé, which is what led to her great anger and sadness.
However, the version of events as presented in XV does not match up with Finé's own version on several points; for example, it is not explained why Finé wants to confess her love to 'God' when God is Shem-Ha, and Finé was already close with Enki before the curse was activated. This discrepancy has yet to be fully explained by any canon material.