The Sacrifice of Natasha Romanoff was the ultimate step that was required in order for the Avengers to obtain the Soul Stone. When the Red Skull informed Black Widow and Hawkeye that they must sacrifice something that they love, the two fought over who would be the one to die, each wanting themselves to be the one, resulting in Black Widow being the casualty.
The Snap perpetrated by Thanos heavily affected the Avengers and their allies who had thrown everything they could in the battle to put an end to Thanos' threat. Among them, due to Captain America and Iron Man taking a few steps back from the team, Black Widow attempted to coordinate the remaining Avengers in keeping safety to the world, hoping to find a way to reverse Thanos' actions. However, she also had to deal with the case of Hawkeye who, having lost his wife and his three children, engaged in a ferocious crusade to eliminate the criminals who had been spared by the randomness of the Snap. In the end, both Black Widow and Hawkeye knew that their efforts only had limited effects.
A glimpse of hope appeared, however, when Tony Stark, based on a idea by Scott Lang, elaborated a Time-Space GPS which could enable the Avengers to get back into the past, collect the six Infinity Stones and reverse Thanos' actions. Following a successful trial by Hawkeye, who had been convinced by Black Widow to return, the Avengers decided to form groups and head into different times and places to find the Stones. Black Widow and Hawkeye where therefore tasked with going to Vormir, where Thanos had previously obtained the Soul Stone by sacrificing his daughter Gamora.
Arriving on Vormir, Black Widow and Hawkeye were greeted by Red Skull, who stated their parents' names, which surprised Black Widow since even she did not know the name of her father. Black Widow and Hawkeye were then told by Red Skull that they had to sacrifice a loved one to obtain the Soul Stone, causing Black Widow to understand why Thanos had left Vormir without Gamora as he had sacrificed her. Black Widow and Hawkeye thus agreed that one of them had to die to return billions of people from death, as they had promised to do whatever it would take to undo Thanos' actions.
However, as they tearfully said their farewell, Black Widow and Hawkeye realized that they were both ready to give their life. Black Widow insisted that everything she had done in the past five years was to find a way to reverse the Snap, and that she should be the one to sacrifice herself to achieve this goal, but Hawkeye countered by recalling that he had become a vengeful vigilante, with the sacrifice being his one chance to redeem himself and bring back his family. However, Black Widow refused to let her friend die, stating that she would not judge him on his worst mistakes just like he had not when encountering her years ago.
Pretending to let Black Widow give her life, Hawkeye suddenly tripped her and pinned her on the ground, asking Black Widow to tell his family that he loved them. However, Black Widow retaliated and immobilized Hawkeye on the ground with her electroshock weapon. She then ran to the cliff to jump, but Hawkeye knocked her aside by shooting an explosive arrow next to her. Hawkeye then rushed to the cliff and jumped, only to be caught by Black Widow who had leaped after him. Black Widow then used Hawkeye's own grappling arrow to secure him to the cliff while she hanged in the air, only held back by Hawkeye's hand. Although Hawkeye refused to let her go, Black Widow pushed herself back with a kick on a cliff and plummeted to the ground, falling to her death just like Gamora had.
Clint Barton suddenly woke up in a pool with the Soul Stone in his hand. Barton mourned over Natasha Romanoff's death before using his remaining Pym Particles to return to the present day. When the other Avengers asked him about why Black Widow did not come back, Barton was very reluctant to answer but explained what had happened to his friend nonetheless. They then mourned her together, and although Thor suggested that they could use the Infinity Stones they had brought to revive Black Widow, Hawkeye insisted that her sacrifice could not be undone. Hulk was also devastated by the loss of Romanoff, and stated that the Avengers had to make her sacrifice worth it.
To that end, the Soul Stone obtained from Romanoff's sacrifice was placed within the Nano Gauntlet along with the other Stones gathered by the Avengers. This eventually resulted in the Blip which successfully brought back everyone who had fallen to the Snap, thus fulfilling Romanoff's wish to do her part to save the world from Thanos' actions.
In the aftermath of the Battle of Earth which ended by the victory of the Avengers and their allies, Barton lamented that he wished to find a way to let Romanoff know that they had won. Wanda Maximoff, who stood by his side, assured him that Romanoff most certainly knew it, just like Vision who had also perished because of Thanos. Hulk also confessed that he had tried to bring Romanoff back when performing the Blip, but that he had failed just as Hawkeye had predicted that her death could not be reversed as it was instrumental in getting the Soul Stone.
Returning to Vormir
Following the Battle of Earth, the Soul Stone was placed in a suitcase and given to Steve Rogers, who traveled back in time to 2014 to return the Soul Stone on Vormir. On Vormir, Rogers encountered Red Skull, although he merely relinquished the Infinity Stone to his old foe without any altercation. Therefore, Red Skull had to remain on Vormir to guard the Soul Stone again.
- In an interview with Russo Brothers, it is revealed that the original script of Avengers: Endgame had Thanos and his forces ambushing Black Widow and Hawkeye as the two fought each other to commit suicide so the Avengers to retrieve the Soul Stone.