The Kauld were a warp-capable civilization of the Sagittarius Star Cluster. Their group identity was referred to as the All Kauld
History and specifics[]
The true origins of the Kauld are unknown but it is known that they share a common ancestry with their rivals, the Blood Many. (TOS - New Earth novels: Challenger, Wagon Train to the Stars)
Unlike their cousins' homeworld, the Kauld planet was geologically stable which led them to become sophisticated and conceited. The stability of the Kauld world led the Blood Many to make contact, landing on their planet and asking them for help. The Kauld refused and the relationship soon became hostile and led to all out war.
Their war consisted in brief skirmishes with sub-light vessels whenever their planets came near one another to then rebuild when they got too far, causing a never-ending stalemate. The Formless, an extra-dimensional species who had performed scientific experiments in a nearby system, took notice of their conflict and decided to give them dynadrive, a form of faster than light engine to broaden their horizons hoping that would lead them to peace. Despite the origin of dynadrive being common knowledge, Vellyngaith, a Kauld general, believed that the benefactor of the Kauld had been the Tholians, not the Formless.
Instead of using dynadrive for exploration the two species used it to attack each other in their native systems leading to a destructive escalation in the conflict as they now had no time to rebuild after a fight.
By the late 23rd century the Kauld military had accidentally contaminated itself with antimatter while experimenting with dynadrive, which pushed them to fight harder for what appeared their impending victory over their long-time enemies. Their situation was further complicated by the arrival of the Belle Terre Colonial Expedition in the Sagittarius Cluster and the discovery of the quasar olivium in the moon orbiting Belle Terre. (TOS - New Earth novel: The Flaming Arrow)
Despite their efforts to destroy the perceived invader their situation became drastically worse when the Blood Many allied themselves with the colony, forcing them to launch a desperate sneak attack on Belle Terre that led to the destruction of the USS Peleliu.
Despite their small victory the Kauld lost all hope for victory when the Cold Factor, a Formless construct created for recovering olivium, became defective and destroyed the planet the Kauld were using to store part of the stolen mineral and directed itself toward the Kauld homeworld, forcing Vellyngaith to surrender himself and his government to the UFPF Challenger and begging the crew for help. Luckily for him both Nick Keller and Shucorion agreed to help in exchange for a stop in all hostilities and when James T. Kirk and the crew of the USS Enterprise managed to stop the machine they offered him McCoy's medical assistance to treat the sick Kauld. (TOS - New Earth novel: Challenger)
Following the end of the war the Kauld retreated to their homeworld to rebuild but some lone rebels refused to surrender and waged a civil war against the Kauld government and harassed the Blood and Belle Terre. (CHA - Gateways novel: Chainmail)
Biology and culture[]
The Kauld were physically similar to the Blood Many, and were blue skinned humanoids with purple blood but unlike them they were extremely proud and wore sophisticated clothes and embellished their hair with pearls and jewels. (TOS - New Earth novels: Challenger, Wagon Train to the Stars)