The Eternal known as Sersi and Dane Whitman (aka the Black Knight) are two of the newest additions to the Marvel Cinematic Universe, and the two have a long, shared history. Having been romantically involved and soul-linked to one another, the two were also members of not only the Eternals, but also the Avengers, and the Heroes for Hire. Their devotion to one another would eventually see the two of them temporarily living in a forgotten part of the Marvel Multiverse known as the 'Ultraverse'.
The now defunct universe was home to a different group of heroes who formed their own superhero team called the Ultraforce, which Black Knight led for a time. But the story of how the two Marvel heroes ended up in an entirely different universe is an interesting one.
The Ultraverse was the premier superhero universe of Malibu Comics, and was home to characters such as Prime, Prototype, Hardcase, and Night Man. The main heroes would eventually set up their own superhero team, Ultraforce, which was their answer to the Avengers or the Justice League. Malibu would eventually be taken over by Marvel and the Ultraverse would become one of Marvel’s many alternative universes. During this time, Ultraforce had their own animated series, and Night Man was the star of a live action series that lasted from 1997-1999.
Following the events of Avengers #375 (by Bob Harras, Steve Epting, and Tom Palmer), Black Knight and Sersi ended up in the Ultraverse after a battle with an evil version of Dane Whitman known as Proctor. Sersi killed Proctor with the Ebony Blade, but the mental torture she had endured under him had caused her to slowly lose her mind. A Watcher then created a portal to another reality for Sersi to enter so that she would have an opportunity to mentally recover, and the Black Knight elected to join her.
While the two ended up on the same world, Black Knight was transported to New York while Sersi resurfaced in Africa. Dane met the Ultraforce and joined their team in Ultraforce#8 (by Marv Wolfman, Hank Kanalz, George Perez, Keith Aiken, and Mark Morales). The following issue revealed that Sersi had been possessed by the seventh Infinity Gem, the Ego Gem, and this set up the Avengers/Ultraforce crossover event known as "Black September."
‘Black September’ revolved around the Ultraforce and Avengers being pitted against each other by Loki, who is trying to win a game against the Grandmaster. Sersi manages to get hold of the other gems during the conflict and they combine to create the new entity known as Nemesis. With the power of all seven infinity gems, doom for the entire multiverse is inevitable, until Black Knight heroically destroys the Ego gem, simultaneously releasing the other six gems. The Avengers and Sersi are then sent back to their original universe with no memories of what happened.
The defeat of Nemesis caused a complete change in reality for the Ultraverse. Characters such as Hardcase and Prototype were either erased or their history was altered. Black Knight, while retaining memories of having lived in another reality, became the leader of the Ultraforce. After stopping an alien invasion, the team discovered that one of the aliens could open portals to other dimensions, and Black Knight and two other Earth-616 natives (Reaper and Siena Blaze) used the alien's power to return to their original home world.
The Ultraverse was already on the decline by this point, as the changes caused by Black September proved unpopular and sales plummeted. In 1997, a one-shot entitled Ultraverse Future Shock (by Marc Paniccia, Fabio Laguna, Kevin West, and Manny Clark) was produced to resolve all of the dangling plotlines of the universe. The Ultraverse and its characters have not been seen or heard from since. Aside from a brief period in 2003 when Marvel was considering bringing the characters back, all signs point to the universe laying dormant for the time being, leaving Black Knight and Sersi as two of the last remaining links to the now-forgotten universe.