The belief that Lord of Chaos and Corruption, Jioni is the true deity who should reign and therefore should be brought to "true life" to rule the world.

Foul Witherer, Lord Darkness, Calamity, King of Darkness

Following the death of Lotea, the goddess' high priestess along with the now scorned believers of her religion were banished to the Undercurrent, to live in exile. With the high priestess blinded and their deity deceased, they tried to regain some sort of foothold, but due to no longer having the resources from the land above, nor the ability to communicate with outsiders, that eventually, their tenacity eventually wore thin.
While they scrounged for resources and any glimmer of hope, they began to change over generations of living underground. The priestess herself gave birth to many new inhabitants, which unfortunately suffered the same fate of becoming partially blind, gangly, losing hair and the ability to reproduce without an aid. Before the followers completely lost all hope and sense, they happened upon a pool of Black Taqa which seemed to almost have sentience, luring them to come closer and listen to its words. Unbeknownst to them, their addled brains desperate for a leader, desperate for a calling, placed their desperation into that pool of murky ink and breathed a conceptual deity into being.
It was Chaos itself, made something close to flesh. Possessing the attention and minds of its new followers, it gave the orders to bring it power, artifacts, magic, anything they could grab to grow its form. The Undercurrent had a new king and his name was Jioni.

Listen to our Lord Jioni.
Do not question your betters.
Fulfill your purpose.
You exist to serve.
Paradise comes to those who listen and obey.

When a pure blood, black haired male Jehkhun is born, it will be the beginning of Jioni's claim to power.
Upon Jioni claiming his new host and body, he will merge with Lotea's remains to become the ultimate God.
With Chaos and Passion combined, Jioni will overthrow Asha and Kasim, bringing an end to their rule.

In the The Undercurrent

Ceremonial chanting in which the attendees hum and chant prayers to Jioni.
Sermon that dictate the current orders from Jioni or from his retainers.
Collection of gathered artifacts and supplies.
Mask removal and adhesion of ceremonial paints
Evening chants and prayers to Jioni.
Monthly ritualistic aphrodisiac consumption and breeding ceremony between the able-bodied.

When not in the cloister or in private chambers, the personalized mask given to members upon reaching adulthood must be worn.
Outsiders must be captured alive and caged until a decision about its fate has been made.
The Oracle is not to be approached without prior authorization.

The false kingdom of Asha and Kasim is in ruins, with Jioni's temple being built over the existing foundation. The congregation is restored to their former forms and they are able to live their lives to Jioni's wishes. Being absorbed into Jioni after death is seen as a peaceful eternal sleep.

Becoming of age and being adorned with the cult's symbol.
Learning the responsibilities for one of the members.