A fal rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 ko runes and a flawed sapphire together to get a fal rune. You can also obtain a fal rune from hell countess. You can get a fal rune from the nightmare hellforge quest.
A fal rune requires level 41 to use.
Horadric cube recipes with fal runes
transmute 3 fal runes and a flawed ruby to create a lem rune
transmute exceptional rare weapon, a fal rune, a um rune, and a perfect sapphire to create an elite rare weapon of the same type