A hel rune is a low tier rune that can be socketed into armor and weapons. You can cube 3 dol runes and a chipped emerald together to get a hel rune. You can also obtain a hel rune from nightmare countess. You can get a hel rune from the nightmare hellforge quest.
A hel rune requires level 33 to use.
Horadric cube recipes with hel runes
transmute 3 hel runes and a chipped diamond to create an io rune
transmute any socketed item, a hel rune, and a scroll of town portal, to remove and destroy all socketed gems, runes, and jewels