A lum rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 io runes and a flawed topaz together to get a lum rune. You can also obtain a lum rune from nightmare countess. You can get a lum rune from the nightmare hellforge quest.
A lum rune requires level 37 to use.
Horadric cube recipes with lum runes
transmute 3 lum runes and a flawed amethyst to create a ko rune
transmute exceptional unique weapon, a lum rune, a pul rune, and a perfect emerald to create an elite unique weapon of the same type