An um rune is a mid tier rune that can be socketed into armor and weapons. You can cube 3 pul runes and a flawed diamond together to get an um rune. You can also obtain an um rune from hell countess. You can get an um rune from the nightmare hellforge quest.
An um rune requires level 47 to use.
Horadric cube recipes with um runes
transmute 3 um runes and a topaz to create a mal rune
transmute exceptional rare weapon, a um rune, a fal rune, and a perfect sapphire to create an elite rare weapon of the same type