Well, the sword was always a "Magical" sword from the beginning. That his father used to defeat Aku the first time. Jack took up the sword and has been using it ever since until the time it breaks (Comic timeline). Then he's forced to roam the dystopian land without his sword and a broken will (being it was his only way to defeat Aku), and Aku attempts to seize the scenario and kill Jack for good. But what eventually happens is a spoiler, so I can't tell you. LOL
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