Sorrow Island is an island that first appeared in "Appetite for Destruction".
The appearance, geology, flora, or fauna (other than dragons) are not seen or discussed for this island.
The Dragon Riders survey dragon populations on several islands. Astrid marks the dragons of Sorrow Islands with yellow paint. As the Riders each mark dragons, they realize that dragons from one island are ending up on the others. They discover that these islands are being systematically destroyed and the dragon populations are migrating. A Screaming Death is responsible, destroying islands as it makes its way to Berk.