Pyramid of Power
"This structure was built to honor the King of the Golden Land. Legends say that Ganon stole the Triforce from a chamber near the top."
"Home to Hidden Treasures"
"Adventurous souls who climbed to the top of the Pyramid of Power during Ganon's reign could observe a once perfect world that had been devastated by a powerful villain. The Triforce, hidden inside, was captured by Ganon and used to bring darkness to the world."
"Ganon is said to have crashed through the top of the Pyramid just before his final battle with the legendary hero, Link."
"A very strong explosive could blast through the cracked wall on this second level of the Pyramid and reveal an unusual Faerie Fountain."
--Nintendo Power Player's Guide
When Link defeats Agahnim for the first time, the evil wizard drags him into the Dark World and he first appears on top of the pyramid. After defeating him for a second time in Ganon's Tower, Ganon appears and turns into a Keese-like creature, flying away. Link follows him to the pyramid using his Flute, where Ganon creates a large hole inside of the top of the pyramid. If Link drops down, he enters a large room with a Triforce on the floor. He then must defeat Ganon, and if he does so, he can exit through a door at the north of the room and claim the Triforce.
Before Link fights Ganon, he can also visit the Cursed Fairy within the pyramid. She gives Link the Silver Arrows, which are one of Ganon's weaknesses, and the Golden Sword. Link needs to use the Super Bomb to destroy the wall blocking the passage to her spring.
In addition, there is another floor directly beneath the room where Link fights Ganon. When Ganon begins to drop floor panels, Link can possibly fall to this second floor. The room contains nothing but a door that leads out on to a ledge in the pyramid, which Link can jump down from to return to the top room.
There is also a Piece of Heart that can be found along the right side of the pyramid.