What is inside of the golden egg in Harry Potter and the Goblet of Fire?

Answer 1

The golden eggs contained disembodied merpeople voicessinging a song to the champions, letting them know that they had to come to the lake and rescue something very valuable to them, which turned out to be a person very close to each of the champions. As the voices were those of merfolk, they could only be understood underwater.

In Harry Potter and the Goblet of Fire, the contestants who successfully beat the first (dragon) challenge, are given golden eggs. Harry is persuaded to open his golden egg, but only a garbled shrieking comes from it. In chapter 25, Harry takes the egg into the prefects' bathroom and Moaning Myrtle suggests that he listen to the egg underwater. He does this, and he hears a song from which he then deduces that he must reclaim something from the mer-people in the lake (the second challenge).