An eth rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 nef runes together to get an eth rune. You can also obtain an eth rune from normal countess. You can get an eth rune from the normal hellforge quest.
An eth rune requires level 15 to use.
Horadric cube recipes with eth runes
transmute 3 eth runes to create an ith rune