This is a Side Quest in Hades (Act IV). Pythes, a guard in the Epirus - Refugee Camp, tells you about the sacking of the City of Paseron by a terrifying army lead by enemy captain Proseia. He believes that the enemy army will be weakend should its leader be defeated.
You can find Pythes next to the rebirth fountain of the Epirus - Refugee Camp. After talking to him, follow the road northwest to Paseron and make you way through the city. Proseia is at the city's northwestern gate, right before the Paseron Outskirts. Kill her and return to Pythes for your reward.