Themis was a 1st generation Titan. Her parents were Uranus (Sky) and Gaia (Mother Earth) and had 12 siblings.
Her siblings were the Cyclops, Hecatoncheires, Mnemosyne, Tethys, Oceanus, Crius, Iapetus, Hyperion, Theia, Coeus, Phoebe, Rhea & Cronus.
After her father was dethroned, her youngest brother, Cronus became ruler. Themis built the Oracle at Delphi and was herself oracular.
According to a legend, Themis received the Oracle at Delphi from Gaia and later gave it to Apollo. She was never wrathful or unfair.
She was present at Apollo’s birth and made him an Oracle too.
She is the mother of life, the Horae, and the Mother of Fate (the Moirai), all sisters.
Themis became a wife, but more importantly, a high counselor to her nephew, ZEUS. After her brother was is defeated, she was allowed to stay with Zeus on Mount Olympus, rather than get sent to Tartarus with many of her brothers and sisters.
When Cronus created the prophecy, Themis was summoned as the Oracle, without his knowledge. She was able to chant caveats into the prophecy allowing for any descendant that has Greek God’s blood in them to go on a quest to solve their part of the prophecy. If all are successful, we win. If not Cronus wins, and the Gods are lost and the earth is plagued.