This is the third mission of a small questline to complete one request of the Kings of Sparta. It needs to be completed to complete The Conqueror.

Like The Fall of Deianeira, this is a simple mission. Find the champion and kill him.

Aristalos is on an island in the middle of Lake Kopais called Kopais Perch. Sail to the island and find him. Upon arrival, he's fighting someone. Don't worry about trying to kill him from a distance--a cutscene will play upon getting close.

The man fighting him is none other than Nikolaos. He kills Aristalos. Gather some information on why he's there and take a moment to confront dear Pater. After this conversation, the mission will come to an end.