Mastered Earth Runes need a Earthstone along with a Blank Rune; a Manasteel Ingot and three Supreme Earth Runes to make one Mastered Earth Rune.
Mastered Runes can be crafted at Arcane Forging level hundred fourty (2-10) at the Rune Altar.
You need three of these to learn Sand Storm and you will need extra to cast the spell.
The command to make a Mastered Earth Rune is [@Neko Chan#7837 rpg rune altar mastered earth]. For more info use [@Neko Chan#7837 item info mastered earth rune].