The Celestials

The Celestials are a doomsday cult who seek to use the Shards of Chaos and the Herald's powers to free Vaatu from the Tree of Time and bring judgement to the world. 

Background

Thousands of years ago, Avatar Wan and Raave sealed Vaatu in the Tree of Time. Before being put away, Vaatu embued each of his twelve Zodiac Heralds with a fraction of his power using the Jewel of Chaos, so they may unite and release him when the time came. 
The Heralds, however, became self-motivated and greedy. They instead used their powers to fulfill individual goals of world domination in the mortal world after they were stranded there when Avatar Wan separated the spirit and mortal world. Avatar Wan would spend his lifetime sealing each of them within their shards of the Jewel of Chaos, except for the Dragon Lion spirit, Mang Kong Sang. 
The Zodiac Heralds, Vaatu, and the Chaos Jewel would be forgotten through the centuries. They would become myths known as the "Zodiac Legends."

Resurgence

At some point, during the era of Avatar Kuruk, a woman by the name of "Mother" learns about the Zodiac Legends. 
At this time, the peace created by Avatar Yangchen was broken by worldwide unrest after Avatar Kuruk dies unexpectedly and the world fails to find the next Avatar in the Earth Kingdom for a decade. Threats like the Peony-Cammelia Civil War in the Fire Nation, the Yellow Neck Uprising in the Earth Kingdom, The Fifth Nation's piracy around the seas, and the appearance of renegade Airbenders convinced Mother that the world is impure and needs a reset.
Mother begins to find people around the world who felt the same as her, and preached about spiritual decay throughout the Four Nations. She slowly developed a gathering, establishing a cult comprised of people around the world. By the era of Avatar Kyoshi, around 293 BG, there estimated size of the Celestials is between five hundred to one thousand members. 

Goals

The Celestials' goal is to gather each of the Shards of Chaos and reform the jewel. From there, she will use its power to remotely free Vaatu from his imprisonment in the Spirit World. Because Avatar Kyoshi is in the early stages of her Avatarhood, she is not established as a threat, which makes this the perfect time to summon Vaatu to destroy Raava once and for all. Otherwise, Vaatu and Raava would not be able to battle until Harmonic Convergence, which will happen a few hundred years from now, during the era of Avatar Korra. 
Mother intends to find suitable, powerful hosts around the world for the Zodiac Heralds to possess so that The Celestials can control world affairs. Once there are twelve hosts to each of the Zodiac Heralds, they will be able to reform the Jewel of Chaos to summon Vaatu and usher judgment into the mortal world. 
The exact goal of the Celestials after the apocalypse is unknown, but many believe their goal is to re-start humanity as its spiritual leaders, molding the world in their vision. 

Membership

The Celestials' base is in an unknown location. People interested in joining the cult must be scouted and recruited by existing members. From there, they undergo a ritual and conditioning before becoming full members. 
Mother has also been rumored to capture benders of unique potential around the world. The sudden disappearance of many children who have been gifted with rare bending abilities (such as blood benders, plant benders, lava benders, combustion benders, etc.) has been linked to the cult. Many believe they are raised as living weapons for Mother's cause. 
Members of the cult live their beliefs in secret, posing as citizens around the world, where they are assigned to live by Mother. They report to the Celestial base and back using Ravenfoxes. Each of them has a different mission and assignment. Members who have been captured are known to swallow a pill to end their life so they cannot be interrogated about The Celestials. 
Many believe that The Celestials have infiltrated governments and organizations around the world. 

Society & Culture

The Celestials are led by Mother, their matriarch. Below her are her "Daughters," four lieutenants who carry her will. Together they are known as the "Daughters of Chaos." 
The Daughters assign missions to members of the cult. Below their rank are specialized benders with unique, powerful abilities. They are known as "Razors." Many have claimed that the Razors have instigated coups, riots, political assassinations, and other nefarious activities around the world. 
Below the Razors are lower ranking members of the Celestials, led by Mother's "Hands." The lowest members are cultists led by the Hands. 
The Celestials are not only comprised of benders. Many non-benders are also a part of the cult, including weapon masters, assassins, brilliant inventors, and more. 
Celestials are known to wear a golden broach that symbolizes Vaatu's eye of judgment over a black cloak when they make themselves known. Otherwise, they blend in with society, carrying out their mission.