Sacrifice of Natasha Romanoff

"In order to take the stone, you must lose that which you love. An everlasting exchange. A soul, for a soul."

- Red Skull to Hawkeye and Black Widow

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 which of the two would die, each wanting themselves to be the one, resulting in Black Widow being the casualty.

Background
As part of their mission to retrieve the Infinity Stones for the Infinity Gauntlet that would bring back the snap victims, Clint Barton and Black Widow were sent back to 2014 with Nebula and War Machine. Eventually, after dropping War Machine and Nebula on Morag to obtain the Power Stone, the two heroes went to Vormir so they could seek the Soul Stone and return to the present day with it.

Sacrifice
"Welcome, Natasha, daughter of Ivan. Welcome, Clint, son of Edith."

- Red Skull to Black Widow and Hawkeye

Upon encountering Red Skull on Vormir, Black Widow suddenly learned that Gamora's sacrifice was the main reason why she didn't return upon being kidnapped by her father. Thanos had essentially traded in her soul so he could get the Soul Stone, as he was required to give up a soul for a soul. Black Widow and Clint Barton fought against each other as both were willing to sacrifice themselves for the Soul Stone, until the two of them ended up of the edge of the cliff, above the rocks where the sacrifice was made. As Black Widow was below Clint Barton's level while hanging off the steep mountain, she decided to let go and sacrifice herself so the Avengers could get the Soul Stone.

Aftermath
"We can't bring her back." "I'm sorry, no offence, but you're a very earthly being, okay, and we're talking about space magic. It can seem very difficult..." "Yeah, look, I know I'm way outside my pay rate here, but she's still dead, isn't she? It can't be undone! Or, at least that's what the red floating guy had to say! Maybe you ought to go talk to him! Okay, go grab your hammer, and you find and talk to him! It was supposed to be me. Sacrificed her life for that goddamned stone, she put her life on it." "She's not coming back. We have to make it worth it. We have to."

- Clint Barton, Thor and Bruce Banner

Clint Barton suddenly woke up in a pool with the Soul Stone in his hand. Barton mourned over 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 didn't come back, Barton was very reluctant to answer.