A thul rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 ort runes together to get a thul rune. You can also obtain a thul rune from nightmare countess. You can get a thul rune from the normal hellforge quest.
A thul rune requires level 23 to use.
Horadric cube recipes with thul runes
transmute 3 thul runes and a chipped topaz to create an amn rune
transmute normal body armor, a thul rune, a tal rune, and a perfect topaz to create a socketed body armor of the same type
transmute normal helm, a thul rune, a ral rune, and a perfect sapphire to create a socketed helm of the same type
transmute normal rare armor, a thul rune, a ral rune, and a perfect amethyst to create an exceptional rare armor of the same type