Attila "desired to make his eldest son their king, and so sent Onegesios to do it". Onegesios returned with Ellac, who "had taken a spill and broken his right hand".
Attila's desire for his eldest son to become king led him to send Onegesios to carry out the task.  Onegesios returned with Ellac, who had been injured in an accident and sustained a broken right hand.