The Federation Starship Avenger was a Defiant class escort. On Stardate 53550.8, Ambassador Worf's Starfleet commission was temporarily reactivated, and he was given command of the Defiant class Starship Avenger; the ship was ordered to Outpost 40 in the Briar Patch so that he could negotiate with the Son'a, who were once again threatening Starfleet's presence in the area. When talks broke down, the Avenger helped defend the outpost from a Son'a attack fleet and successfully repelled them.
Shortly after repelling the attack, Worf received word of an impending Borg invasion fleet and was ordered back to the Klingon Empire to gather support. Realising that time was of the essence, Worf ordered the Avenger into the dangerous Ikolis Expanse and was soon ambushed by a small task force led by Toral, who had created a fake copy of the Sword of Kahless and intended to kill Worf.
Heavily outgunned, the Avenger was forced to plot an erratic course through the Expanse and use derelict vessels to aid in its escape. Once clear of the region, Worf proceeded into the Gamma Quadrant to retrieve the real Sword of Kahless, and returned to Qo'noS. When the Avenger arrived in orbit of Qo'noS, they discovered that Chancellor Martok was under siege from Toral's forces. Both Martok and Worf suspected that Toral was receiving outside help and Martok had the Avenger fitted with a Cloaking device so they could expose Toral's conspirators.
After following Toral's ship following an unsuccessful attack, the Avenger discovered that Toral had been receiving support from the Romulans. After reporting this to Martok, the Avenger left to join the main Federation fleet in defending Sector 001, leaving Martok to attack the Romulan base.
As the battle raged in the Sol system, the Avenger travelled into the Delta Quadrant with a combined Klingon/Romulan fleet, and successfully captured a Borg construction vessel. Using this vessel, a Transwarp gate was created, and the U.S.S. Enterprise NCC-1701-E led the allied fleet into the Delta Quadrant. Together, the fleet was able to destroy an Omega molecule that the Borg had created and escaped back to the Alpha Quadrant.
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