In order to get this quest you have to do all the previous quests for Demosthenes and also for Lysander and then talk again to Demosthenes.
Warning: You can only complete a quest for one of the sides (Spartans or Athenians). The rewards in both quests are the same. The alternative quest is Nemesis.
General Demosthenes asked the Eagle Bearer to fight on behalf of Athens at the battle of Megaris.
