A nef rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 tir runes together to get a nef rune. You can also obtain a nef rune from normal countess. You can get a nef rune from the normal hellforge quest.
A nef rune requires level 13 to use.
Horadric cube recipes with nef runes
transmute 3 nef runes to create an eth rune