Warp to the Mirror Chamber, turn into your human form, climb the steps and the Mirror will be put together. The sages will appear and say they were not able to stop Ganondorf. You will learn more about Midna's history and intents, who is in fact the Twilight Princess! She realizes that her world and the world of light are linked together. Go behind the mirror, climb the magic stairs and enter the Twilight Realm.
Palace of Twilight
Get to the centre, follow the path on the right and open the door.
West part
Destroy the plants and avoid the energy balls, then get close to the enemy looking like Zant and hit it with your sword several times to get rid of it. Then pick up the Small Key in the chest, shoot the Clawshot at the targets and unlock the door to the north.
Turn into a wolf to go across the darkness (this transformation is forced) and attack another Zant's head, which makes two chests appear. Backtrack a little and pick up the Compass in a chest to the southwest. Then go back north and pick up another Small Key in the second chest. Now shoot your Clawshot at the western target, then at the one on the ceiling and let go. Go south and find 100 rupees/Angry Midna stamp in the chest. Then latch on to a target to the north, let go and unlock the door to the north.
Go to the back and a fight will start. Get rid of the different enemies, but don't lose focus on Zant and strike him around ten times to beat him. Go north of the room and Midna will tell you more about the Sols. Swing your sword at the hand holding this Sol, then quickly pick it up, place it in a recess in the middle of the room, then quickly climb the magic stairs, shoot your Clawshot at the Sol to get it back and open the south door while carrying it.
Still hurrying as the hand is chasing you, go to the middle of the room, drop the Sol in the recess, then climb the magic stairs and pick up the Sol using the Clawshot before the hand picks it up. Go on to the south, throw the Sol, then climb, pick it up and open the door to the south. Get through the room towards south while avoiding enemies and open the door.
Get close to the Twili to give them back their normal form, then place the Sol in one of the recesses in the centre of the big platform. Then step on the light square that will take you to the west. Open the door.
East part
Shoot your Clawshot at a target right or left, then at the one on the ceiling. Let go on the moving platform, then jump to the next platforms until you reach the north while using your shield to block the head's attacks. Fight Xant's head, then pick up the Small Key in the chest to the west. Also pick up 50 rupees in the chest to the east, then unlock the door to the north.
Jump in the middle of the room and use Midna's energy ring to catch all three Twilight Beasts. Then attack the three Zant's heads while avoiding their energy balls to make a chest appear. Go north, turn back into your human form and shoot the Clawshot at the closest target, then at a target on the ceiling. Go down using the chain and take a look at the room. Use the many targets to find 50 rupees/Zant stamp in the chest northeast, 50 rupees in a chest northwest, the Map in a chest east and finally a Small Key in a chest west. Go back north and hook onto a target to climb up, then unlock the next door.
Like you did before, get rid of two or three plants, then focus on Zant and strike him with your sword, even if he is faster this time. You can wait for him to prepare an attack so you can rush towards him. Once he vanishes, hit the hand to the north, carry the Sol and run to put it in the middle of the room. Climb the magic stairs, use the Clawshot to draw the Sol to you, then open the door to the south. Place the Sol in the middle of the room, then come back north. Shoot the Clawshot at the target to the north, then at the one on the ceiling, then at the one to the west side, then at the one on the ceiling towards the south, then at the one to the southwest and let go. Climb the stairs and go south, then use the Clawshot to pick up the Sol. Jump to the south and open the door.
Here, always keep the Sol with you. Climb on the platform and go across on the next platforms to the south. Once you arrived, another platform shall appear between the two spheres on the ground. Stand on it and go through the room to the south door. (If you ever fall down, go to the sphere on the ground to the north to make a new platform appear which will enable you to go back up.) Open the door.
Outside, get on the light square to go back to the centre and drop the Sol in the second recess. At this moment, the Master Sword will be filled with Light and will now have the ability to cut through dark fog!
Now go back on the path heading east and open the door. Go ahead, then look right at the small passageway and swing your sword to clear the fog. Shoot the Clawshot at the target to the top and open the chest to find a Piece of Heart! Get out, go back to the west using the square platform and open the door.
Go left, cut through the fog using your sword and climb on the light platform on the ground. Jump to the chest when the platform is at its highest point and open it to find another Piece of Heart! Jump towards the entrance, then go right and do the same to reach a chest containing 50 rupees. Jump to the entrance, shoot the Clawshot at the targets and let go on the first moving platform, then jump towards the two spheres. Do a spin attack to switch them on and go back south. Open the door and go back to the centre.
Central part
Go straight ahead, get rid of the Kargaroks, then do a spin attack on the red drawing on the ground. Quickly cross the gap before the darkness comes back and open the next door.
Get rid of the enemies and swing your sword to clear the darkness. Pick up a Sol, place it on a recess in the middle of the room, then do the same with the other one. Climb the newly-appeared stairs, hit the sphere with your sword, then get on the moving platform. Jump to the other side, get rid of all Zant's heads, then pick up the Small Key in the chest. Come back a little, stand between the three spheres and do a spin attack to switch all three of them in one hit and reveal a new moving platform. Go across on this platform towards the third floor and unlock the door.
Do a spin attack in front of the dark fog to the west, then jump. Fight the Kargaroks, then shoot the Clawshot at the target to the north, then at the one to the south, then to the north, to the south and again to the north to climb to the top. Let go and open a chest to find the Big Key! Jump down and go through the fog once more. Go east and get rid of all the enemies around. Hit the two spheres to the east, stand on the moving platform leading you south and strike Xant's head. Then step back on the plaform to go back north, move north and defeat all the other Xant's heads to make another chest appear. Open it to get a Small Key, then unlock the door to the northwest.
Go a little further, then when the Twilight Beasts appear, turn into a wolf and use the energy ring to get rid of them in one run. Go back to your human form, swing your sword to clear the darkness, then do a spin attack between all four spheres to switch them all on. Stand on the platform, then jump on the next one to the east, then on the northeast one and jump on the ledge with a plant. Clear it out, then jump on the platform to the northeast and when it gets close to the west wall, shoot the Clawshot at the target on this wall. Get rid of Xant's head, then latch on to the target on the ceiling and let go on the moving platform. When you are getting close to the east wall, shoot the Clawshot at the target to the east, then get rid of the other Xant's head to make a chest appear. Jump towards this chest southeast, and pick up another Small Key. Then jump north, jump on the moving platform aside, which leads to some stairs north to the fourth floor. Unlock the door.
Go ahead and attack all Twilight Beasts, one by one for the first ones, and fight the gathered ones using your spin attack, which will clear the darkness in the room. Open the door to the boss.
Usurper King: Zant
After his little speech where we learn that he wanted to be the next ruler and discover he got tricked by a so-called god, the fight can now begin. As his name induces, he will impersonate previous bosses' attacks.
The first step is the Forest Temple: While avoiding his attacks or using your shield, aim at him using the Z lock and use the Gale Boomerang to draw him close to you, then quickly hit him with your sword. Do this once more so he changes his tactics.
Second step, Goron Mines: Run towards him without falling from the platform, avoid his energy balls or use your shield, then as soon as his attack is over, strike him a few times with your sword. Do this once more to get to the next step.
Third step, Lakebed Temple: Put on your Zora Armor and your Iron Boots to reach the bottom of the water, then if needed, break some pots to refill your hearts. Then as soon as his attack stops, aim at him with Z lock and use the Clawshot to draw him at you and hit him with your sword. Then he will take cover in one of the four shelters. Once more, get close to him while avoiding his balls, then when his attack is over, use your Clawshot to draw him at you and slash him.Fourth step, Forest Temple (mini-boss): Refill your hearts by breaking some pots if needed. Look at him jump from pole to pole, then get close to him when he's attacking while dodging his balls. Ram twice into the pole to make him fall, then quickly strike him several times. Do this once more.
Fifth step, Snowpeak Ruins: Run while looking at the ceiling using the frozen ground so he doesn't fall onto you, then as soon as he falls, face him, use the Z Lock and aim at one of his feet with the Ball and Chain. He will shrink, quickly run after him and strike him with your sword. Do this twice more to reach the final step.
Sixth step (never seen before): During this step, be really attentive towards Zant's noises to know what he's doing without seeing him. Hit him with your sword when he swings his arms at you, then get away when he's spinning around, and as soon as he stops, run towards him and slash him. Do this twice more so he will surrender.
Back to the palace, Zant will make some important revelations about Ganondorf, his master, and then Midna will use her magic to pierce through him! Pick up the Heart Container on top of the stairs, then go back to Midna to get out.
Outside, go to the light circle to the far south near the magic screen and press A to go back to Hyrule and save the princess!