An eld rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 el runes together to get an eld rune. You can also obtain an eld rune from normal countess. You can get an eld rune from the normal hellforge quest.
An eld rune requires level 11 to use.
Horadric cube recipes with eld runes
transmute 3 eld runes to create a tir rune
transmute low quality weapon, an eld rune, and a chipped gem to create normal quality weapon of the same type