The Mexica/Aztec god of the underworld, Mictlantecuhtli, has began stealing blood sacrifices from the other gods, culminating in a Zapotec city being leveled. To these ends, Quetzalcoatl decides to close the gates to the underworld, but can't due to said gates being made of obsidian, which is toxic against the gods.
So instead he makes a bet with Tezcatlipoca: he will find a human, "the lowest of the low", and crown him his champion so he will close the gates before the equinox, when Tezcatlipoca will hug off humanity. Tezcatlipoca agrees to the bet, but sends his champion Yaotl along as an emissary so Quetzalcoatl doesn't cheat. Quetzalcoatl chooses Izel, a slave who lost his sister to blood sacrifice.
Onyx Equinox Ended after Season 1
Crunchyroll aired the final episode on Friday 1st January 2021