The Xylith is a small plant and the Undines use its leaves to make a transparent material from which they build the bubbles in which they live. The only known place where it grows is the back of a giant turtle that lived in the Black Mud Swamp and where there were many Red Willows.
The Xylith grew only on the back of an island. which was in reality a giant sleeping turtle, in the middle of the Black Mud Swamp and the Undines who went there to take Xylith leaves home would fall asleep because of the leader Red Willow tree. Because of this and the movements of the turtle's head in its sleep, the Undines believed that there was a monster in the swamp that attacked the Undines.
An Undine named Lusiz went on the island to take Xylith leaves but fell asleep because of the leader Red Willow. When the Winx went to the Black Mud Swamp to help Flora prepare for her exams, they met the Undines and Queen Algae told them about the Xylith and how risky it is to collect some from the island. She also tells them about Lusiz and how she never came back. The Winx then decide to go to the island and defeat the monster.
After Bloom and Flora defeated the leader Red Willow, the turtle woke up again and swam away. The Undines thought they would have to leave their homes and become vulnerable to predators but Flora used a magic potion she made to make many new Xylith leaves grow for the Undines so they will always be safe in their homes.