Thunderhead

The Thunderhead is a location in The Sky in Skyward Sword. It is a giant, billowing storm cloud near Skyloft. Around the outside of Thunderhead, tornadoes are common. On the inside of Thunderhead, plenty of lightning flashes occur, though they eventually stop once the parasite Bilocyte is defeated. The Sky Spirit, Levias, inhabits the area, watching over the Sky. The Thunderhead houses the Isle of Songs, Bug Rock, and several other islands, some of which contain Goddess Chests. Although it is noted that the Thunderhead appeared suddenly in the Sky, there are no further details regarding its origin.

History
Once the mirror is activated on top of the Light Tower, it reflects a beam of green energy that parts the clouds covering the entrance. This allows Link to access the Thunderhead and enter the Isle of Songs, which houses three songs that the young hero must learn: "Farore's Courage", "Nayru's Wisdom" and "Din's Power". Later, Link must defeat Bilocyte, the parasite infecting the Sky Spirit Levias, to learn about the four parts of "Song of the Hero" needed to find the Triforce. After the young hero learns the first three parts from the three Sacred Dragons in the Surface, he must come back to the Thunderheard to learn the final part of the "Song of the Hero."