Serpent-tongues are mentioned once in How to Seize a Dragon's Jewel, at the commencement of the battle at Prison Darkheart. "Serpent-tongues writhed and twisted around the white dragon bones, shaking out their wet wings. . . . The darkest types of dragon species, the monsters, the ones that hated humans the most." ―How to Seize a Dragon's Jewel Serpent-tongues are clearly part of the Dragon Rebellion, against the humans.