Dark Runes need a Darkstone along with a Blank Rune; a Silver Ingot and three Weak Dark Runes to make one Dark Rune.
Runes can be crafted at Arcane Forging 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 Dark Rune is [@Neko Chan#7837 rpg rune altar dark]. For more info use [@Neko Chan#7837 item info dark rune].