Return to Hyrule Castle and make your way to the Elemental Sanctuary. This is the same area where you first saw the Picori Blade. With all four Elements, approach the pedestal. The Elements will combine with the Picori Blade, transforming it into the legendary Four Sword. This powerful weapon allows Link to create up to three clones of himself, a crucial ability for solving puzzles and defeating enemies in the final dungeon.
Immediately after reforging the sword, you'll witness Vaati's true form and his takeover of Hyrule Castle. The castle transforms into a dark, ominous fortress. This event marks the beginning of the final act of the game. Take a moment to understand the Four Sword's mechanics; you can create clones by standing on specific glowing panels and pressing the L button. Experiment with different formations, as they are essential for navigating Dark Hyrule Castle and defeating Vaati.