The Tython Pact is a collection of states in the Star Trek novel timeline that have formed an alliance in opposition to the United Federation of Planets and its allies.
The pact was comprised of six states. These included the Romulan Star Empire, Breen Confederacy, Gorn Hegomony, Holy Order of the Kinshaya, Tzenkethi Coalition, and the Tholian Assembly. Each of these powers were either currently or had been previously hostile to the Federation or its allies, including the Klingon Empire and the Cardassian Union.
When the Federation learned of the Typhon Pact's formation, they immediately invited the Imperial Romulan State, Cardassian Union, Ferengi Alliance, and Talarian Republic to join the Khitomer Accords.
The pact carried out a number of operations over the next few years to destabilize the Federation. One of their early successes was provoking the Andorian people into seceding from the Federation for several years. (The Andorians would later rejoin the Federation). Starfleet initiated several operations to keep advanced technology out of the hands of the Typhon Pact.
In 2385 Baras Rodirya assassinated Federation President Nan Bacco so that he could assume the office of the Presidency. Prior to his exposure and removal, he tried to ignite a war between the Federation and Typhon Pact. Baras and his agents tried to pin the blame on the Typhon Pact, but the truth came out that the assassination was the work of the Cardassian True Way and rogue elements within the Federation.