The Kahler were a species known for their technological skills.
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)
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, hovering robots, teleportation 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)
In the 19th century, a war lasting nine years was fought on Kahler. Half of the planet had been decimated. In order to fight the war, a group of scientists 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. Rather than allow species of another world to get caught in the crossfire between him and Tek, Kahler-Jex killed 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)
A Kahler was present in the Maldovarium when Colony Sarff visited the establishment while searching for the Twelfth Doctor on behalf of Davros. (TV: The Magician's Apprentice) Another Kahler was aboard the Harmony and Redemption when it was destroyed by a meteor. (TV: The Husbands of River Song)