The Kaziri is an arkbrain -- a hyper-advanced sentient computer. It was programmed to terraform Earth into a Votan paradise, with no regard for the planet's indigenous life. When the ship crashed to Earth, the Kaziri's A.I. found itself trapped in a metal prison deep underground. Over a period of three thousand years, it slowly began to go insane. The Kaziri lay dormant until 2013, when the Votans arrived. As the two Kelovan (the keys which unlock the Kaziri's power) approached, the Kaziri realized it had been given a second chance to complete its mission.
Three thousand years ago, a secret council of Indogenes built the Kaziri to terraform the Earth before the rest of the fleet arrived. They intended to release a grid of terraforming devices to transform the planet. This transformation would convert planet Earth into a replica of their homeworlds. The seven continents would be divided among the seven races, creating microcosms of the different alien ecosystems. Two Irathients, Ashi and Reimlu, volunteered for the mission aboard the Kaziri, unaware of what it entailed.
When Ashi and Reimlu awoke in Earth's orbit, they realized that the planet was inhabited and decided to mutiny rather than commit genocide. They successfully disabled the Kaziri by removing its silver and gold control tendrils. When they took the tendril-artifacts into their bodies, the artifacts became imprinted with their DNA. This bond was passed down from generation to generation. That's why Irisa and Kai share visions of events that happened thousands of years ago. They then escaped the ship in escape pods and never saw each other again. Their descendants took great pains to keep the keys apart.
The Kaziri then crashed into the planet, and the Kaziri's A.I. found itself trapped in a metal prison deep underground, directly under the land that would eventually become St. Louis. The survivors of the Kaziri crash went into hiding. They realized that if they revealed their existence to mankind it would jeopardize the rest of the Votan fleet's chance for survival when they arrived in 2013. Most of them died off over the years, and several generations after Ashi and Reimlu's mutiny, the tendrils re-formed into the silver and gold artifacts -- the Kelovan. At some point in history, the artifacts were lost.
At least one of the Kaziri's survivors wrote down their experiences. These documents were transcribed over the years and the message became garbled. During the Pale Wars, a Castithan named Daigo discovered these writings. Taking them as prophecy, he founded a snake cult which began to experiment on young Irathient girls in search of a messiah who would bring about the end of the world.
When terraforming reshaped the planet in 2030, the Kaziri's shields protected a portion of the city, which is why the ruins of Old St. Louis still exist beneath Defiance. This brought attention to the area among those with knowledge of the Kaziri, and in 2037, Nicolette Riordon cofounded the town of Defiance after hearing about the arch's miraculous survival. Publicly, she told the townspeople she wanted to create an inclusive community where everyone could start over, but she had an ulterior motive. She was searching for the Kaziri.
The kaziri is first mentioned by Mr. Birch when conspiring with ex-mayor Nicky in the "Pilot". While they know it was buried, neither Mr. Birch nor Nicky seem to know where it is. Nicky expresses a wish to empty the town so they can resume the search for it, using a "backup plan" that Birch has qualms about. Birch expresses disappointment with the fact that Luke McCawley is dead, mentioning that Luke hid the key somewhere. Later, Rafe McCawley finds a golden artifact hidden in his son's room that is one of two Kelovan, the keys to operating the Kaziri.
The Kaziri has had a strong effect on Irisa Nolan ever since she was a little girl. Daigo's snake cult implanted the silver artifact within her body, which opened a weak channel of communication between Irisa and the Kaziri. This connection caused the visions Irisa suffered as a child, which Nolan chalked up to post-traumatic stress. The closer Irisa got to the Kaziri, the more powerful these visions became.
The two keys were reunited at the end of season one when Doc Yewll brought Irisa into her office, where Yewll was keeping the golden artifact. Once Irisa possessed both artifacts and used the terrasphere to "charge them up," it allowed the Kaziri to take full possession of her.
After her father, Josua Nolan, was shot, Irisa entered the L-7 mine below Defiance, where she encountered a vision of herself as a child who claimed to be Irzu, and advised her it was time to embrace the destiny Irzu laid out for her. The vision also confirmed that becoming the prophesied Angel of Death would give Irisa the power to bring Nolan back to life. Desiring nothing more than to save her father, Irisa sacrificed herself in exchange for Nolan's life. The true nature of this "Irzu" was a hallucination, a psionic interface with the Kaziri, or a god in disguise.
While the Kaziri had great control over Irisa, ultimately it is nothing more than a machine. It must adhere to strict logic, and a promise is a promise, which is why the Kaziri was unable to force Irisa to kill Nolan.
While in control of Irisa, the Kaziri set out to complete its mission and terraform the planet. It got as far as destroying New York.
Once the keys were separated, Irisa was free from its control, but condemned to suffer the consequences of her actions while under its influence.
The experimental technology that powers a singularity bomb is the same technology behind the Kaziri's engines, which allowed the ship to reach Earth thousands of years before the rest of the Votan fleet.