|“||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 mentioned once in How to Seize a Dragon's Jewel, at the commencement of the battle at Prison Darkheart.
Serpent-tongues are clearly part of the Dragon Rebellion, against the humans.
There is no information about the Serpent-tongue dragons. According to the passage above, they have (presumably functional) wings, and live under the sands of the Amber Slavelands. The verbs 'writhed' and 'twisted' imply a serpentine body shape, which might be on the smaller end of the dragon size spectrum, since Serpent-tongues are twisting about other dragons' bones.
Serpent-tongues appear at the attack on Prison Darkheart against the humans there.