Soldeen was a giant fish-like monster that resided in the Lake of Tears and guarded the Ruby. He was Nanion, the leader of D'Or, transformed and trapped in the Lake by Thaegan's magic. Later when Thaegan was destroyed, Soldeen transformed back to Nanion.
100 years ago, the sorceress Thaegan, who hated all things beautiful and good, transformed the city of D'Or into the Lake of Tears and its people into the monsters that inhabit it. The cheiftan, Nanion, she turned into an aquatic serpent and renamed Soldeen. He had no memories of his previous life.
After the Shadow Lord invaded Deltora, the great Ruby was placed on the Weeping Rock—which was Nanion's wife Ethena trapped in stone. Even though the ruby was dulled by the evil around it, the gem brought happiness to Soldeen.
Lief, Barda, Jasmine, and Manus were attacked by Soldeen when they came to the Lake of Tears. He easily tossed them about before returning to the deeper parts of the lake. On the shore, Manus played his flute to help cheer the companions. Soldeen heard the music and rose from the depths of the Lake to speak to the travellers. He explained that he knew they were there for the Ruby, but would not let them take the one thing that brought him joy. He offered to make a deal: trade the Ruby for Manus, who would play his flute on the Weeping Rock until he died.
The Ralad man was willing to make the sacrifice, but his friends refused to make a trade and Soldeen attacked again. Lief managed to grab a hold of one of his spines and threatened to stab Soldeen in the eye if he did not let them go. Touched by their dedication for a friend, Soldeen suddenly found himself remembering his life as Nanion (thanks to the power of the Topaz). With his memories restored, Soldeen ferried Lief to the Weeping Rock and allowed him to take the Ruby. As Soldeen and Lief made their way back to the shore, Thaegan arrived and blasted Soldeen with her magic, stunning him. He was transformed back into Nanion when Kree killed Thaegan.
Soldeen was a tortured soul, constantly weighed down by the sorrows that the Lake of Tears instilled. He was desperate for anything to bring him joy and happiness and would fight ferociously to acquire and defend it.
Soldeen could breathe water and air. He would crawl on the shore and attack enemies or chase his prey. Soldeen also had the ability to speak because he spoke to Lief, Barda, Jasmine and Manus when they camped near the Lake of Tears.