During a time travel adventure, the Avenger Mockingbird was kidnapped, drugged and raped by the Phantom Rider. Mockingbird sought vengeance and after a fight allowed her tormentor to fall to his death rather than help him up from a cliff's edge. Discovering that Mockingbird had allowed a man to die, her husband Hawkeye kicked her out of the Avengers. Tigra and Moon Knight (at that time entirely possessed by Khonshu) subsequently also left after siding with Mockingbird's plight. The trio formed their own team of Avengers shortly afterwards.
The "ex-Avengers" were created by Steve Englehart and Al Milgrom and first appeared in West Coast Avengers #37. While Englehart had plans for the team, including Black Goliath joining them and an eventually confrontation between the "Ex-Avengers" and the real Avengers, these plans never came to fruition as Englehart quit the book a couple of issues later due to editorial disagreements concerning his character Mantis (his dialogue was being rewritten and stories changed at the last moment). As a result, many of his strands he introduced in his final issues never really came to much. His re-introduced Black Goliath was written away in a fill-in issue and the "Ex-Avengers" team broke up after a few more fill-in issues with Tigra returning to the Avengers immediately afterwards without any explanation. While Englehart has been very vocal and open about what he intended for his Mantis stories, much of what he had planned for Mockingbird's team he hasn't spoke of.
The team's most notable moment was when they were featured prominently in the Evolutionary War crossover where they were joined by (their would-be member) Black Goliath. In one fill-in issue, the team were also joined by Daimon Hellstrom. Nevertheless, the team dissolved fairly quickly with Mockingbird and Tigra returning to the real Avengers and Moon Knight quitting altogether when Khonshu released his hold over him.
Major Story Arcs
During the High Evolutionary's war on humanity, the Ex-Avengers discovered the H.E.'s base and were subsequently captured. Together with Black Goliath they fought the H.E. who opted to flee instead of staying and fighting the group. Shortly afterwards, Hawkeye's Avengers arrived at the spot where the base had once been only to discover they were too late and left wondering what had happened there.
The team fought the Night Shift group shortly afterwards and their member Digger was arrested. The remainder of the team retaliated by attacking the real Avengers believing that the trio who attacked them were still members.
The team's last active adventure had them, alongside Hellstorm (again without explanation), battling the spirit of Mockingbird's tormentor, the Phantom Rider. After the spirit of the Rider was exorcized, Moon Knight was likewise exorcized of Khonshu's possession and sharply quit the team altogether stating he had no desire to ever be an Avenger and it was Khonshu's desire and never his. Mockingbird also opt to leave to work out her things and what she is going to do about her break-up with Hawkeye. As the remaining members Tigra, and apparently Hellstorm, pondered what they will do next, Hellstorm reminds them that they are Avengers (even though he's... not).
In an alternate reality where the High Evolutionary won the Evolutionary War, Mockingbird and Moon Knight did not become "godlike ones" as they were only humans. Tigra and Black Golaith were among the beings who left Earth.