(This will be short because I don't have a lot for his lore, sorry :( )
Tul was an Ascended like Nasus and Renekton, an Emperor of Shurima that didn't saw how beautiful Runeterra actually is. When he ascended it was told to him that he could do what he wanted, even if it was returning to be a human. With this in mind he really wanted to travel around Runeterra and learn more about the other places. When he did this he ordered a Shurima Shaman to transform him to a human again, and so it was done. When Tul was back he had the desire to learn the ways of the Shaman, because he was actually pretty impressed with what he did. That Shaman transformed a "god" into a simple human beign and he wanted to know how. When teached the Shaman told Tul that he must travel around Runeterra to seek more knowledge about the other places and so, he would gain more experience as a Shaman. As Tul traveled he met nice people and teached them his ways and in return they would teach their ways to him. He travelled all Runeterra and so he felt prepared to return home. Finally when he was almost to arrive Shurima he watched the new Emperor, Azir, was about to ascend. But then when he was watching he noticed that something was strange and he had to be quick to arrive and stop what was going to happen. Xerath, Azir's best friend, pushed him a side so he could ascend. Suddenly Tul only saw sand in front of him, every single Shurima citizen was turned to sand, every building turned to sand. While he could he gathered some of the souls of Shurima citizens to help him on his long life journey of trying to ressurect them all. And one soul in special was with him, the Master Shaman that teached him.