The Legend of Zelda: A Link to the Past/Agahnim's Tower

Preperation
Even though you have all three Pendants, you can't just storm Agahnim's Tower and take him on. You need to do a little bit of prep work first. Let's get started.

Link and the Master Sword
You have heard the tales of the power of the legendary Master Sword, and you know that you need to fulfill your quest. Back in the time of the Imprisoning Wars, the people of Hyrule had forged a Sword that could withstand the forces of magic. One had to be pure of heart to wield the blade. Sahasrahla, the village elder, instructed you on how to obtain the Master Sword. He said that only a hero who collects the three Pendants can take the Master Sword. You may have encountered the Master Sword in the Lost Woods before you collected every Pendant. It is planted in a stone with mysterious Hylian script on it. After defeating Moldorm in the Tower of Hera, return to the forest grove and claim the Master Sword. Now you can continue with your quest to seek out and destroy the evil power in Hyrule Castle, the wizard known as Agahnim.

Discovery of the Ether
The message on the Hylian Monolith at the top of Death Mountain, to the west of the Tower of Hera, instructed readers to return with the Master Sword. Now that you have the blade, you may wish to return to the Monolith to heed the advice. Return afteryour visit to the Lost Woods, and the monument will yield up the powerful magic spell of Ether. After that, the monument will crumble. You will use the magic of Ether to open dungeons in later stages of your quest.

Breaking and entering at Agahnim's Tower
Early in your adventure, you stormed Hyrule Castle to save Princess Zelda. During that time, you may have discovered a door on the roof that was barred by a bolt of lightning. Any method that you may have tried to break the Lightning Lock failed. Now that you have the Master Sword, you may have better luck. If you strike the Lightning Lock with the Master Sword, the lightning will vanish and Aganhim's Tower lays open.

The Castle Tower
Sahasrahla dares you to retrieve three Pendants. Each was guarded by a fierce creature, so the challenge was not to be taken lightly. Sahasrahla rewarded you when you returned with the first Pendant by telling you about the Master Sword, which only proven adventurers could wield. Once armed with the Master Sword, you can challenge Agahnim in the Castle Tower. The entrance to the castle tower is sealed with a force field that only the Master Sword can break.