Rune Shells appears in Suikoden IV.
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Rune shells are ammunitions for the Rune Cannons. They contain elemental forms of Runes (Fire, Wind, Water, Earth, Lightning) that were used in naval battles during the Island Nations War. Warlock was the one who created them. He managed to find a way using the seeds of the Giant Tree and combining them with elemental runes, to create those powerful ammunitions. These Rune shells can't be used without a Rune Cannon.