The Dagger was a criminal syndicate located in the Abantis Islands of Greece during the Peloponnesian War. Using blackmail and bribery to control the people and economies of Skyros and Euboea, the group was founded and led by a man calling himself 'the Kingfisher'. They were also supported by the Cult of Kosmos.
During the Peloponnesian War, the Dagger kidnapped a farmer who wouldn't pay them tax. The Spartan misthios Kassandra was tasked with finding the farmer by his wife, Rhoeo. The slave Agapios, acting on the orders of his magistrate master, later employed Kassandra to find out the true identity of the Kingfisher and clear suspicion from his believed-dead brother, Neritos.
A smuggler working for the Dagger was using the local performers, Prexileos and Idyia, to spread the Dagger's propaganda via their theater and the children in the couple's care. The smuggler initially paid them for their services, but then ceased, threatening to come after the children if they stopped. Prexileos wanted the man dead and would have killed him himself if not for Idyia persuading him not to. Kassandra later put a stop to the smuggler.
The Dagger controlled ships as well, and one of them was sunk by a slave master around 431 BCE. Though the slave master perished as well, one of his slaves escaped to the nearby ruins of Eretria and lived to watch the Dagger circling the shipwreck like sharks. When Kassandra met the former slave, she decided to explore the shipwreck and found an unusual dagger within.
Kassandra's investigations found that Agapios' brother, Neritos was in fact the Kingfisher; the founder and leader of the Dagger. Kassandra and the Magistrate rushed to confront Neritos at the Temple of Achilles on Skyros, only to find Agapios trying to reason with his brother. In the ensuing confrontation, Kassandra managed to kill Neritos, as well as the rest of the leading figures of The Dagger, thus ending the organization.