In 431 BCE, Hyrkanos was hired by Athens to aid their forces in Megaris against the encroaching Spartan forces. A master of manipulation, Hyrkanos tended to delegate duties so as to ensure his own safety. To this end, he hired an Athenian soldier and gave him a Spartan soldier outfit in order to infiltrate the Spartan camp in Pagai and steal a tablet containing an account of their activities in the region.
At one point, Hyrkanos also attacked and slew a Spartan supply convoy on its way through the Occupied Forest of Tripodiskos. The supplies he left alone, letting the citizens of Megara, among them Neaera, take them.  The Spartans sent Dolios to investigate, and he enlisted the help of the misthios Kassandra to hunt down and eliminate Hyrkanos, and she did.
Behind the scenes
If Hyrkanos is not slain, he's encountered and slain during the Conquest Battle in the quest "The Final Push."
Otherwise, Hyrkanos can be recruited as a lieutenant for the Adrestia with a unique avatar.
Recruiting or defeating Hyrkanos before The Final Push also reduces the nation power of Megaris down by 100 unless the nation power is already below 100, in which case it will drop to 0 from whatever number. This is unique in the sense that no other mercenary contributes to the downfall of nation power at any subsequent point in the game.