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