In The Voyage of the Dawn Treader, Eustace becomes a dragon on Dragon Island. After the Dawn Treader is damaged in a storm, the crew lands there to make repairs, while Eustace wanders away to avoid helping.
He discovers the lair of a dead dragon and greedily sleeps on its treasure while wearing a golden bracelet. The magic of the hoard transforms him into a dragon. The bracelet belonged to Lord Octesian, one of the seven Narnian lords who disappeared during an earlier voyage, leading Caspian to suspect that the dragon may have been connected to Octesian's fate.
Eustace's transformation becomes a turning point in his character. As a selfish and unpleasant boy, he initially resembles the dragon he has become; as a dragon, he recognizes the harm caused by his pride and greed. Aslan eventually helps him shed the dragon form, painfully removing the enchanted skin and restoring him to humanity. The crew later names the island Dragon Island because of what happened there.