A ko rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 lum runes and a flawed amethyst together to get a ko rune. You can also obtain a ko rune from nightmare countess. You can get a ko rune from the nightmare hellforge quest.
A ko rune requires level 39 to use.
Horadric cube recipes with ko runes
transmute 3 ko runes and a flawed sapphire to create a fal rune
transmute exceptional unique armor, a ko rune, a lem rune, and a perfect diamond to create an elite unique armor of the same type