Crescent Island is a volcanic isle in the eastern Fire Nation that houses a temple for the Fire Sages, an mportant location in the Avatar's spiritual training.
As aptly named, the island was crescent shaped, and aside from its active volcano, the most notable feature was the Fire Temple. The Fire Temple was a five-tiered pagoda that sat on a hill near the volcano's base, managed by a small group of Fire Sages. The ancient temple used mechanisms that required firebending to operate. Within the temple was a room called the sanctuary, which was vital for firebending born Avatars to master the Avatar state. The chamber was largely empty and devoid of most light, save for a giant ruby that upon the Winter Solstice, the light of the sun aligned directly with it and filled the dark chamber with light, a spiritual moment helping to trigger the Avatar state.