Combat uses an Action Time Battle (ATB)-style system where characters have a recharging store of energy to use for special actions. Another way to view it is like a store of Mana for magical attacks, where the Mana gradually recharges.
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Basic attack elemental damage - inflicting elemental damage recharges OP
Energy shards - inflicting damage can dislodge energy shards that land on the ground and can be collected to recharge OP.