The moon's road is a path that forms on the surface of the ocean, typically accessible to those with faerie blood. The path is a crystalline, glass-like path that leads out to sea, formed by the moonlight and indicated when the reflection of the moon on the water seems solid. The path is transparent and moves with the waves of the ocean beneath it.
After a time, the path dissolves. The moon's road will crack and break until it is completely gone.
Julian Blackthorn, Emma Carstairs, and Cristina Rosales followed Mark to the moon's road and caught up with him near the Gate, using Cristina's medallion, which was enchanted with faerie magic, to use the road despite not possessing faerie blood. They then encountered the Guardian of Lir's Gate, a phouka who collected a toll from each of them in exchange for a promise. Once the Gate opened, the moon's road began to disappear and the Shadowhunters passed through the Gate just before it completely dissolved.