The Kahler were a species known for their technological skills.
Biology
Kahler looked identical to humans except for green patterns on their faces. These patterns were unique and could be used to identify an individual. (TV: A Town Called Mercy)
Culture
Individuals were always identified with a "Kahler" in front of their name. The Kahler had gods and believed that after dying, they would climb a mountain, carrying the souls of everyone they wronged. (TV: A Town Called Mercy)
Technology
The Kahler were known for their technology; the Eleventh Doctor called them "one of the most ingenious species in the Galaxy". They had egg-shaped spacecrafts, had teleports and could easily cure cholera.
During a war in the 19th century, they created an army of cyborgs using Kahler soldiers. One arm was replaced with a powerful energy weapon, one eye was replaced with a lens and much of the body was replaced with robotic parts. This eye had an automatic targeting system, sensors that could see through walls, and identification software. They also possessed a short range teleport. (TV: A Town Called Mercy)
History
In the 19th century, the Kahler were at war with an unknown enemy. In order to fight them, a group of scientist modified Kahler into cyborg soldiers. With them, the war was ended in a week. The soldiers were decommissioned, but one, Kahler-Tek, went rogue and killed all the scientists responsible. Two scientists, Kahler-Jex and Kahler-Mas, escaped but crashed on Earth.
In 1870, Kahler-Tek killed Kahler-Mas and tracked Kahler-Jex to Mercy, Nevada. He was unwilling to kill the innocent inhabitants, so he trapped them in the town and demanded Kahler-Jex's return. Once the Eleventh Doctor arrived and learned their history, he first tried to give Kahler-Jex to Kahler-Tek, but eventually decided to help him escape. Kahler-Jex instead decided to spare the town and kill himself with the self-destruct on his ship. Following the urging of the Doctor, Kahler-Tek stayed in Mercy as a protector. (TV: A Town Called Mercy)