A pul rune is a mid tier rune that can be socketed into armor and weapons. You can cube 3 lem runes and a flawed emerald together to get a pul rune. You can also obtain a pul rune from hell countess. You can get a pul rune from the nightmare hellforge quest.
A pul rune requires level 45 to use.
Horadric cube recipes with pul runes
transmute 3 pul runes and a flawed diamond to create an um rune
transmute exceptional unique weapon, a pul rune, a lum rune, and a perfect emerald to create an elite unique weapon of the same type
transmute exceptional rare armor, a pul rune, a ko rune, and a perfect aamethyst to create an elite rare armor of the same type