A lem rune is a mid tier rune that can be socketed into armor and weapons. You can cube 3 fal runes and a flawed ruby together to get a lem rune. You can also obtain a lem rune from hell countess. You can get a lem rune from the nightmare hellforge quest.
A lem rune requires level 43 to use.
Horadric cube recipes with lem runes
transmute 3 lem runes and a flawed emerald to create a pul rune
transmute exceptional unique armor, a lem rune, a ko rune, and a perfect diamond to create an elite unique armor of the same type