In the latest MCU production Eternals, Gemma Chan plays Sersi while Kit Harington appears as Black Knight aka Dane Whitman. Both characters have shared an intense and tragic romance in the comics, especially in Bob Harras' tenure on Avengers in the mid-1990s.
While Sersi has always been an Eternal with transmutation and matter-altering powers, Black Knight was introduced as a human character who was proficient in combat skills, swordsmanship, and horseriding. They ended up falling in love during their time with the Avengers. However, a love triangle, mental connections, and alternate dimensions spelled doom for their romance.
Black Knight Also Loved Crystal
Whitman was attached to Sersi when the Avengers were fighting off the villain called Proctor. Around that time he also harbored a temporary fixation with the new Avenger recruit and Inhuman Crystal.
In Avengers #361, Whitman made his feelings for Crystal more obvious as he was disappointed with Ikaris creating an intimate mental link between him and Sersi. However, he gave up his feelings for the Avenger and their relationship never materialized as Crystal had her own struggles because of a rocky marriage with Quicksilver. The latter was her estranged husband but tried reconciling with her by Avengers #375. Regardless of his temporary distractions, Black Knight ended up going back to Sersi and re-igniting their romance.
Members Of The Ultraforce
The Ultraverse was an alternate universe under Malibu Comics that featured the superhero team called Ultraforce. Following an accidental portal, Black Knight and Sersi ended up arriving in the Ultraverse.
In the Ultraforce comics, Sersi ends up in this universe's Equatorial Africa while Whitman reaches Miami. Possessed by the Ego Gem, Sersi wreaks havoc against the titular superhero collective. On the other hand, Whitman joins the team alongside other "ultra"' like Prime, Topaz, and Hardcase. When they discover a larger conspiracy involving Loki and his pursuit of the Infinity Stones, the Avengers and Ultraforce come together to ward off the possessed Sersi's attacks. This is yet another instance that shows Sersi's immense powers can make her a pawn in the hands of evil forces. In such situations, Whitman faces a dilemma as he must save both the universe and the love of his life.
Ikaris Created A Mental Link Between The Two
During one of Sersi's unstable bouts, the Eternals feared that she was suffering from the Mahd W'yry disease. This condition is present in some of the most powerful members of the Eternals due to their extended life periods. In a last attempt to save her, Sprite suggested that Black Knight should be her Gann Josin. For those who are unacquainted with this concept, a Gann Josin basically refers to soulmates that are telepathically connected.
Without seeking Knight's consent, Ikaris immediately formed this bond between the two lovers as it would stabilize Sersi's condition. Knight was extremely disappointed with this move as this happened during a tumultuous phase of their relationship when he had started developing feelings for Crystal.
An Evil Black Knight
The Proctor was a mysterious villain who drew the ire of Earth-616's Avengers when he formed a team called the Gatherers. This collective included evil versions of the Avengers from multiple dimensions.
Proctor seemed to have a particular fascination with Sersi who was only growing more uncontrollable with her powers. Finally, in Avengers #374, the big secret comes out as Proctor reveals himself to be an alternate version of Black Knight. He also reveals that it was he who was responsible for Sersi's seizures and Mahd W'yry disease. In his own dimension, Proctor's advances were rejected by Sersi. Angered by this and possessed by his Ebony Blade's curse, he killed her and set out on an inter-dimensional crusade to kill all other Sersi's of the multiverse. Freed from Proctor's control, Sersi fights back and kills him with his own Ebony Blade.
Teleported To The 12th Century
Following their adventures in the Ultraverse, Black Knight and Sersi attempted to get back to Earth-616 albeit in a different timeline. Finding himself in the 12th century, Dane Whitman finds his soul to be trapped in the body of his ancestor Sir Eobar Garrington. Under this guise, Whitman not only fights one of the X-Men's best villains Apocalypse but also Sir Garrington's best friend Bennet du Paris (who would later go on to become the mutant Exodus).
Apocalypse puts du Paris's body in a comatose state when he refuses to go against his friend. With their intention being survival, Whitman and Sersi avoid any further conflict. Sersi forms another portal and this time, the couple gets teleported to the right timeline. This adventure from Black Knight: Exodus #1 proves that both characters are willing to stick with each other in times of need.
Black Knight Was Possessed By Sersi To Fight The Avengers
With Ikaris setting up the Gann Josin link between Black Knight and Sersi, the Eternals hoped that Sersi's maddened state would be reduced to normalcy. However, the situation worsened as Sersi ended up taking control of Black Knight's brain and turned him towards her side.
After destroying the Avengers Mansion in Avengers #374, she and Black Knight confronted the Avengers at Brooklyn Bridge. However, when he confronts Crystal, one of the most powerful Inhumans, Black Knight snaps back to his normal state. Shocked by his feelings towards Crystal, Sersi's anger increases and she destroys the Bridge too. The destructive act shows Sersi's emotions towards a confused Whitman who seems to be so madly in love with Crystal that he ends up losing the telepathic link he had with Sersi.
The Split After Onslaught
Their 12th-century detour eventually led Black Knight and Sersi to a present timeline where the Avengers have been killed by the villain Onslaught. However, when Knight reached New York, Sersi was nowhere to be seen. The former Avenger grieved the death of his fellow superheroes in Heroes For Hire #2-3. The latter parts of the issues also found him encountering the Lady of the Lake who granted him enhanced powers and the Sword of Light.
Armed with these new gifts, Black Knight eventually joined Heroes for Hire. Sersi finally reappeared in his life and sought his help to foil one of her old foe Ghaur's villainous schemes. Even though Knight complied, the couple finally split after the mission. No specific reason was given but for readers, it is quite obvious that their romance was always an ill-fated one. The two never seemed to reconcile again with Sersi finding new romantic partners like Makkari.