Auzou Book for the unicorn master tome 6 medusa-city

Since Aëlig and Kalyane disappeared, Illana is inconsolable... Accompanied by Hiyama, she decides to explore the depths of the ocean to find her friends who might have survived... They then discover a strange city under the sea, inhabited by creatures that are half-seahorse, half-human. Could they have something to do with the disappearance of Aëlig and the unicorn?