Normal Dark Runes need a Darkstone along with a Blank Rune; a Silver Ingot and three Weak Dark Runes to make one Normal Dark Rune.
Normal Runes can be crafted at level ten at the Rune Altar.
You need three of these to learn Dark Slash and you will need extra to cast the spell.
The command to make a Normal Dark Rune is [nya!rpg rune altar dark]. For more info use [nya!info normal air rune].