Ancient Avengers Assemble
After the Rider achieved revenge against Wendigo, Lady Phoenix and Odin asked him to join them. Eventually, they were joined by the Sorcerer Supreme Agamotto, Iron Fist Fan Fei, the first Black Panther and the Starbrand-wielder Vnn. Though they came to trust one another, the group did not always see eye-to-eye on everything and oftentimes rubbed one another the wrong way. According to Odin's memory, Agamotto was arrogant, Starbrand and the Rider were often at each others' throats, and the King of Asgard never quite liked Fan Fei or her home nation of K'un-Lun.
The Avengers of 1,000,000 B.C. found themselves facing a threat with roots going back four billion years ago when an Alpha Celestial known as the Progenitor came to Earth to die, infected by space locusts called the Horde. As it bled into the molten surface, the Celestial gave parts of its cosmic self to the planet, which would help explain the myriad of astonishing individuals who would come from that previously insignificant place. In the team's era, another Celestial, Zgreb the Aspirant, came to Earth looking for its love, the Progenitor. Zgreb was also infected by the Horde and potentially driven mad seeing its lover's lonely place of dying.
In its madness, Zgreb was attacked by the Stone Age Avengers and killed. Though some regretted murdering the strange visitor, they ultimately decided to bury the body under what would eventually be known as Africa, though Odin wanted to nail the body to the moon as a sign to other potential invaders. This action brought about a visit from the First Celestial Host, a group investigating the deaths of their brethren. The Avengers attacked and were swiftly felled, but then the Host simply left, leaving them to ponder their actions.
Time Trouble
As the prominent super-squad of the time, this group of primordial Avengers experienced their fair share of time travelers, including some unnamed evil-doers. Some teammates briefly witnessed a battle between the time traveling Lockjaw, D-Man, and Annihilus while others met Thor's far-future granddaughters Frigg, Ellisiv and Atli as they sought out Avengers through time.
The group had a much more prominent visitor when Iron Man found himself transported to this era by a mysterious cave found in modern-day Turkey. Presuming him to be like the "armored despots and futuristic conquerors" they had already faced, the team attacked. The battle was brief, but Stark remained there for months, often tormented and tempted by Mephisto. The ancient Avengers returned later to witness Tony fight the devil who also possessed a Time Stone, which allowed Tony to return to his own time.
Tales To Astonish
Most of the Ancient Avengers' exploits remain lost to—or hidden by—time, but many have been briefly mentioned by Odin over the years and glimpsed. It has been revealed that they came close to killing the primordial mother of vampires, but Odin was talked out of this plan. The group also found itself in conflict with Khonshu's Earthly minions as well as a young Thanos convinced to attack the planet by Mephisto. During a bit of downtime, the team traveled to Wakanda where Black Panther treated his team to his nation's hospitality and delicacies.
A Legacy for the Ages
It's currently unknown how this group came to an end, but in many ways Odin and his ilk have kept the group alive. The old god keeps statues of his comrades in his personal chambers and still has a special place in his heart for Firehair. Odin would also tell their tales back on Asgard where a young Loki Laufeyson, AKA Loki, would hear and remember these words. Further, both Thor and his aforementioned far-future granddaughters would lead groups made up of similar entities in their respective times including the Sorcerer Supreme, Iron Fist, Starbrand, Black Panther, Spirit of Vengeance, Phoenix host and an Asgardian leader.
In the modern era, Loki—who inadvertently created the modern Avengers—decided to dig Zgreb up only to find that the Horde inside had turned him into a Dark Celestial. Loki began a plan to bring the Final Host to Earth which was stopped by a new group of Avengers who had a bit of assistance from Odin. He would go on to pass information to the likes of Ghost Rider (Reyes) and Black Panther (T'Challa) based on his relationships with their predecessors.