The Ancient of War - Act 1 Side Quest
Start Location: Spartan War-Camp
Quest Location: Pellena Valley
Objective: Speak with Hippias then return to Euthycles
The Ancient of War side quest begins by speaking with Euthycles in the Spartan War-Camp during Act 1. For this side quest all you need to do is speak with Hippias and then return to Euthycles to turn it in. You'll find Hippias northeast of the Spartan War-Camp, not too far from the location you fight Nessus for the Main Story Quest.
If you're having trouble locating Hippias I recommend you use the screenshots below for some additional guidance.