Strong Fire Runes need a Sunstone along with a Blank Rune; a Silver Ingot and three Fire Runes to make one Strong Fire Rune.
Strong Runes can be crafted at Arcane Forging level twenty at the Rune Altar.
You need three of these to learn Burning Inferno and you will need extra to cast the spell.
The command to make a Strong Fire Rune is [nya!rpg rune altar strong fire]. For more info use [nya!item info strong fire rune].