The vast Kriosian Empire was once ruled by two brothers, named Krios and Valt, from the ancient Temple of Akadar on Valt Minor. They would become divided over the love for a woman, an empathic metamorph, known as Garuth. They both loved Garuth with such passion that the Empire fell. Krios eventually kidnapped Garuth from their homeworld and took her to the planet that became his namesake. This action started a centuries long war between the two worlds and subdivided the species into the Valtese and the Kriosians. (TNG: "The Perfect Mate")
By the 22nd century, the Kriosians were under the rule of the Sovereign Dynasty of Krios Prime, a family known on hundreds of worlds. The royal guardsmen of the Dynasty were known as the Sovereign Guard. Their leader was known as the First Monarch.
In September 2152, the future First Monarch Kaitaama was returning from a diplomatic mission when the Retellians attacked her transport. They killed her guards, placed her in stasis, and attempted to hold her for ransom. En route to their destination, the stasis pod where she was held aboard the kidnappers' ship began to malfunction.
They were assisted by Enterprise, which later discovered the truth behind the kidnapping when their own chief engineer, Charles Tucker III was himself kidnapped. Both Tucker and Kaitaama were later rescued, and the First Monarch was transported back to her homeworld. Kaitaama later ascended to the throne around May 16, 2153. (ENT: "Precious Cargo")
In 2367, the Kriosians began fighting for independence from the Klingon Empire. During the revolt, the rebels hid in the Ikalian asteroid belt, where they used the actinides in the asteroids to hide from sensor scans. The rebels later attacked a Ferengi freighter and a Cardassian freighter, in the belt.
For the most part, the Klingons tolerated the rebellion, until Vagh, the Klingon governor of Krios, charged the Federation with supplying the rebels with phaser rifles. Upon further investigation by Starfleet, it was discovered that the suppliers were, in fact, the Romulans, who were planting the evidence in attempt to drive a wedge between the Klingons and the Federation, and to jeopardize the Treaty of Alliance. (TNG: "The Mind's Eye")
The following year, Krios and Valt Minor finally agreed to the Ceremony of Reconciliation, to finally bring an end to their centuries long war. The Ceremony was held aboard the Federation starship USS Enterprise-D at a point mid-way between the two systems.
Kamala, the first female metamorph born on Krios since the mid-23rd century, was bred for mating to Chancellor Alrik, the leader of Valt Minor. Leaders of both worlds, in the hopes of restoring peace, prearranged this event at the time of her birth. Kept in stasis during the journey, Kamala was the key to peace between the two worlds.
The Ferengi became involved when they learned of the treaty, attempting in their own right, to acquire Kamala. They approached Briam, feigning the wish to have exclusive rights to transport all Kriosian products to the Valt system. Eventually their involvement lead to the premature waking of Kamala, and a near fatal accident with Kriosian ambassador Briam. Following the accident, Jean-Luc Picard was chosen to negotiate in his place in attempt to represent Kriosian interests of peace.
Despite receiving the gift of Kamala, Alrik was more concerned about the remaining provisions in the treaty, including unacceptable aspects of proposed trade agreements, the review of uncommitted territories, and the schedule for exchange of technology.
Ultimately the negotiations and preceding Ceremony were successful. The wedding, held in a holodeck recreation of the Temple of Akadar, went as planned, as Krios and Valt Minor took their first steps towards peace. (TNG: "The Perfect Mate")
Physically, Kriosians are almost identical to their ancestors, the Valtese, possessing a smattering of spots along the side of their face, that lead to their back, and come to a point about halfway down their spine.
Kriosians are also known for their somewhat randomly born empathic metamorphs. While male metamorphs are somewhat common, female metamorphs only appear once every seven generations and are highly sought after as mates. These metamorphs also experience the Finiis'ral, a life stage similar to late-stage Human puberty. During its end, a metamorph would morph and imprint for the final time, a process that took about two days, as the metamorph would reach absolute sexual maturity. (TNG: "The Perfect Mate")
Society and Culture
The Kriosians had a structured class system, divided between the nobility and the commoners.
Female metamorphs are commonly taken from their parents by the age of four and placed in a royal village where they are prepared by their tutors and servants. These attendants would constantly educate the metamorphs in literature, history, art and sex. (TNG: "The Perfect Mate")
Kriosian culture during the 22nd century did not allow the First Monarch to socialize with the opposite sex. If anyone attempted to "make a pass" at the Monarch, the Sovereign Guard would cut off one of their hands. (ENT: "Precious Cargo")