Follow the corridor, enter the room and the fight against the Calamity begins...
The four Champions appear and direct their respective Divine Beast's power at Ganon, which makes him lose half of his lifebar.
Under a giant spider form, Ganon uses the attacks of the four Ganon Blights, notably throwing fire, a huge sword, whirlwinds, ice cubes, blue lazer beams and charged-up pillars at you. He also uses the Guardians' beam. Avoid his attacks, destroy the ice cubes, and when you can, shoot Bomb Arrows at his head. But the best is to try to send back his fireballs or his blue rays at him by raising your shield at the right moment. If you succeed, Ganon falls down stunned, you can then rush at him and hit him as much as you can with the Master Sword or another strong weapon. When he has lost over half of his initial lifebar, the second phase begins.
During this phase, Ganon shields himself under a fire wall, against which none of your attacks is effective. So you have to keep on avoiding his attacks the best you can and send him back his fireballs with the best defense you can get. You can also use Urbosa's Fury when he's on the ground and preparing an attack requiring a few seconds to charge. When he's hit by his own attack or Urbosa's thunder, his barrier temporarily breaks. Rush at him, put on some of the Barbarian's clothes if you have some and maybe eat a mighty meal to raise your attack. Then hit him continuously until he's completly up. Repeat these steps two or three times to get rid of him.
But Ganon hasn't said his last word. He leaves the castle and takes the form of...
Dark Beast Ganon
You end up near him, your horse by your side, and Zelda sends you the Bow of Light. Gallop to the bow to automatically pick it up, then select it from your pouch. Gallop around him, then when Zelda makes you notice the shiny lights on his body, slow down a little, raise your bow and shoot arrows at these three shiny dots on one side of his body. You have unlimited arrows, so don't hesitate. Once the three targets have been hit, keep galloping around him, then three lights appear on the other side of his body. Hit them the same way, then gallop under his body and shoot an arrow at the light on his belly. Then, Ganon shows his weak point, an eye, such as the ones in the Divine Beasts, on his forehead. Get off your horse, then enter one of the whirlwinds, fly up and shoot one last arrow at this eye to finally defeat him!
Then, Zelda joins you and tells you she was with you for your whole journey. On the castle's roof, the King of Hyrule and the Champions look kindly at you two. Hyrule Kingdom is saved!
After watching the ending credits, you start again before the final fight. To leave the castle, fly to the south until you leave the castle's walls, then you can warp wherever you want.
If you found all the memories, an additional sequence starts after the scene with the King of Hyrule and the Champions. Zelda and yourself are in the Hyrule Field in front of the castle, and the princess tells you about her intention to go visit the Zora King to talk to him about his late daughter Mipha.