The Thornlands is an area added by Twilight Forest. This biome acts as a barrier to the Final Plateau, surrounding the aforementioned biome. In appearance, the top layer is made of Weathered Deadrock, followed by a few more layers of Cracked Deadrock and the rest is Deadrock, but the Thornlands grows a large, dense bush of Thorns. These bushes are decorated with Thorn Leaves and Thorn Roses. If the player attempts to cut their way through the thorns, new Green Thorns will grow in place, simply making the thorns more tangled. The only way to traverse through the Thornlands safely is to locate a Lamp of Cinders in a Giant Obsidian Vault.
This biome is protected by the Twilight Forest Progression line, and is unlocked alongside the Highlands once the player has slain the Snow Queen (prior to 3.7.424), or the three bosses, the Hydra, Ur-Ghast, and Snow Queen, have been slain (as of 3.7.424). If the player attempts to enter the Thornlands before unlocking the area, they will experience Acid Rain that causes damage over time.