Trunks ultimately defeats Fused Zamasu, leading to Goku Black's demise.
While Goku, Vegeta, and Future Trunks all fight against Goku Black and Zamasu throughout the "Future Trunks Saga" in Dragon Ball Super, the final blow that leads to the eradication of Goku Black comes from Future Trunks.
Here's a breakdown:
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The Zamasu Problem: The primary antagonists are Zamasu and Goku Black (Zamasu in Goku's stolen body). They eventually fuse into Fused Zamasu, becoming incredibly powerful and difficult to defeat.
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Trunks' Final Stand: After numerous failed attempts by Goku and Vegeta, Future Trunks gathers the energy of the remaining living beings to create a Spirit Bomb-infused sword.
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The Eradication: With this newly acquired power, Trunks slices Fused Zamasu in half. While this doesn't initially kill Zamasu (due to his immortality and the spreading of his essence throughout the universe), it does lead to Zeno erasing the entire timeline, effectively ending Goku Black's existence as he was a part of the fused being and the timeline itself.
Therefore, while Goku, Vegeta, and Zeno play important roles, Trunks' decisive act is the one that effectively leads to Goku Black's permanent end.
