Winx Club Forever
When the Black Willow left Lulia it settled inside a cave found in a mountain and started weeping in memory of its sister. Its tears formed a pond, and they started flowing out of the cave and up the cliffside, forming the Water Stairway, a waterfall which flows upside down since its water flows upwards instead of flowing downwards like other waterfalls. Its water has the ability of reversing time and some of it was used to heal Faragonda after Valtor trapped her in a tree in Season 3.
When the Winx girls went to Linphea to find a way to make Faragonda return to her normal appearance after she had been merged with a tree by Valtor, they met the Sage of Linphea. The Sage told them that the only way of helping Faragonda was to reverse the flow of time since Valtor's magic was too powerful.
The Sage then told them the story of the Black Willow. According to her, aeons ago, two willow trees were the source of life of the lush region of Lulia, a lush prosperous land north of Linphea. Flora replied that Lulia is a wasteland, and the Sage told her that Lulia was not so before the Great Realm Fire which totally destroyed one of the willow trees and badly damaged the other, which became known as the Black Willow. Its grief was such that it could not stay where its sister had once stood, so it left Lulia. It then went to a cave in a mountain found near the Sage's residence, where it began weeping for its lost sister. Its tears soon became a puddle which then became a pond. flowing out of the cave and up the cliff-side, defying gravity and forming what is now known in Linphea as the Water Stairway. When Aisha expressed her surprise at learning about a place where the water flows backwards, the Sage told her the memory is what makes the Black Willow cry, and that is why its tears could reverse the flow of time and change what has happened.
The Winx then decided that if they could get some of the water and water the Faragonda-tree and turn back time, and bring her and the tree Valtor merged her with just before Valtor's spell was cast, they would then be able to free her. The Sage however warned them that they must act quickly, or else Valtor's spell would become irreversible if they wait too long, and that they needed a goblet to collect the tears of the Willow as touching it would turn them back into children.
When the Winx went to collect the tears of the Willow, they were attacked by the Trix and Darcy used her powers to ruin the environment where the willow is found, and corrupt the Willow and the river formed by its tears.
When Flora obtained her Enchantix after falling into the then polluted water of the Water Stairway, she used her Fairy Dust to heal the Black Willow, which caught the Trix with its branches and threw them into the river which had also been healed. The water of the river turned the Trix into children, and the Winx collected the tears of the Black Willow and restored Faragonda to her original appearance using the tears.