He ran away from Whiterun when he was seven, because the Legion slaughtered his entire family. He traveled the lands of Skyrim alone. He had a friend who lived in Morrowind but visited Skyrim to get him food, but the Legion found out and sent an assassin who killed his friend, much to the dismay of Throno.
One day he met a man named Crom whilst he rested underneath a tree. Which later revealed to be on the border of Crom's camp. Crom asked Thonro if he was sent to kill him, and if he is he a member of the Legion. The answers to both being no, he now travels with Crom and a man named Haaki as their guardian.