The Ascended are individuals that have been Chosen by the Eleven. While they themselves are not gods, they are something stands between the two. Before one becomes Ascended, they must first be Chosen. And this Choosing Rite, the way that one is selected to become an Ascended, is dependent on which of the Eleven chose them. To be Chosen is considered a deep honor, not a burden. They are individuals granted supernatural powers and abilities by the gods, and while they were once duty-bound to protect the world from monsters known as Spirit Beasts, they now roam the lands, free to choose their destiny and make their own fate.
The Ascended are inextricably linked to the Eleven. Depending on which of the Eleven chose the individual, they are granted specific boons and powers. Most importantly, however, only the Ascended are capable of truly killing Spirit Beasts, should they return.
The most powerful and legendary amongst them are the First Ascended, heroes of a forgotten age that protected Auria from great threats— forces that would have brought about the end of all life and the ultimate destruction of the world. They are the first of Ascended to have ever existed, to have ever been blessed with the powers of the gods. Among the First Ascended was Corva, who eventually became a member of the Eleven through an Apotheosis Rite.
Mechanically, Ascended replace the Backgrounds system of Fifth Edition, though you are still more than welcome to use the themes, traits, personality aspects of the standard backgrounds. Note that the Background features of the Ascended are going to appear "unbalanced" in relation to other 5e Backgrounds. Each of the Ascended were designed with the intent of only being used on Auria or a similarly high-magic, high-religion, supernatural setting.