Forbidden Island is one of several islands in the lake neighboring the city of First Warren in Natalia. Unlike its neighbors, it is a barren, rocky landmass known to inspire fear among the younger denizens of First Warren.
Following the Dragon War, Lander plans for his followers build a dam in order to create Lake Merle. This process will transform a series of mountain peaks into Forbidden Island and its six neighboring islands. Lander also has the dragon hatchery on the island converted into the Tomb of Lander's Dragons.
Helmer mentions that he and his brother-in-law Snoden swam to the island in their youth, though the two got in trouble as a result. Heather Longtreader is thrown into a dark cavern after being stabbed by Garten Longtreader. Within the cave, she finds Smalls, who had also been wounded and thrown into the cave. After the Battle for First Warren, Picket sees the island and is struck with longing for Heather, unaware of her presence on the island.
On returning from Harbone Citadel, Picket takes note of the island and wonders why it was forbidden in the first place. Heather and Smalls' efforts to destroy the Dragon Tomb result in Forbidden Island collapsing, even as Lake Merle is drained following the destruction of First Warren's dam. Fortunately, the Seventh District fleet escaping Akolan arrives in time to rescue the pair.