Death, also known as Rio Vidal, is a fictional character portrayed by Aubrey Plaza in the Marvel Cinematic Universe (MCU) media franchise based on the Marvel Comics character of the same name. As the personification of death, she is immortal. Besides her status as a cosmic entity, the MCU and Marvel Comics characters differ greatly. Likenesses of Death appeared in the MCU films Guardians of the Galaxy and Thor: Love and Thunder, before being portrayed for the first time by Plaza in the Disney+ series Agatha All Along. Death was one of the entities who created the Infinity Stones. As Rio Vidal, Death is described as the original green witch; she can take a human form and was in a relationship with Agatha Harkness. After Agatha hid from her, Death sought to deliver Agatha's death and played along as part of Agatha's coven during the events of Agatha All Along.

Death was brought into existence at the beginning of time. She is one of the four cosmic entities who created the Infinity Stones, thousands of years before the 21st century, along with Eternity, Infinity and Entropy. She is recognized for her cosmic power across the universe, being depicted in effigy form on the vault of a temple protecting an Infinity Stone on the abandoned planet Morag, and as a bronze statue at the Gates of Eternity. Death, in non-human form, appeared to divination witch Lilia Calderu when she was a young woman in the 1500s.

Some time before 1750, Death, as green witch Rio Vidal, entered a relationship with Agatha Harkness, whom she had met while collecting the dead from one of Agatha's killing sprees. The pair lived together. Death, as Rio, can speak Spanish. When Agatha went into labor in 1750 with her son, Nicholas Scratch, Death appeared; Agatha bargained with her over the life of Nicholas, and Death granted them more time. Nicholas was an ill child, and Death took him when he was six years old – though Nicholas was happy to go with her, she made him say goodbye to a sleeping Agatha before taking him. Death may have been Nicholas' other parent; she later expressed regret over taking him, saying it is something she did not want, but had, to do. This affected her relationship with Agatha, whom she hurt, with Death later referring to Agatha as "[her] scar".

Agatha then acquired power to be able to hide from Death, using the Darkhold. Death tracked Agatha down to Westview, New Jersey, in 2026, after Wanda Maximoff stripped Agatha of her power and the Darkhold. When Death found her, Agatha was under the effects of a spell cast by Wanda, trapping her mind in an assumed reality. Death was able to infiltrate the spell to appear as FBI agent Rio Vidal and coax Agatha to remember her real identity now that Wanda was "gone". With a powerless Agatha having broken free from the spell, Death tried to fight her, but Agatha implored Death to wait until she regained power to make it a better fight. Death decided to tell the Salem Seven, a rogue witch coven who wanted to kill Agatha, of her location.

After the death of Sharon Davis, Agatha and her coven summoned a green witch to the Witches' Road; Death crawled out of Sharon's grave. Death knew that the Witches' Road was not real, though at first did not know why it had appeared, but commented on its aesthetic and joined the coven as they walked it, introducing herself as green witch Rio Vidal. Her appearance concerned Billy Maximoff, who had been present for her earlier fight with Agatha, but she was welcomed by the rest of the coven: she had initially scared them with her introduction, but they wanted a green witch and were happy to be joined by someone whose presence annoyed Agatha. Still incognito as Rio, Death joined the coven in the second trial, where Agatha tried to get the coven to reject her by tricking her into talking about taking their bodies in a recording booth with the intercom secretly switched on. The conversation, which also revealed Agatha would steal the other witches' power, was quickly forgotten when Billy triggered a curse. In breaking the curse, Billy was injured, and afterwards Agatha told Death to leave him alone; she "mysteriously watche[d] from afar" as the rest of the coven tried to heal him, which Jennifer Kale succeeded in doing. The coven then bonded over discussing scars, with Death describing how she unintentionally hurt Agatha and that this still affected her; the revelation caused Agatha to walk away from the group. Death followed and comforted her, but would not allow Agatha to kiss her, and told Agatha that Billy is not hers.

Death stuck with the coven as they ran from the Salem Seven and entered the third trial. The trial involved a ouija board that initially revealed the presence of Death in the room, causing her to laugh, though this was brushed off as Agatha interfering with it to avoid the trial. In the trial, Agatha was possessed by the ghost of her dead mother, Evanora Harkness, towards whom Death expressed vitriol. Alice Wu-Gulliver intervened to exorcise Evanora from Agatha's body, but in the process, Agatha siphoned Alice's powers, killing her. Death remained in the trial when the coven left, staying to guide Alice to the afterlife.

She did not rejoin the coven, but did appear as Death to reveal herself to Lilia during Lilia's final moments, which also revealed Rio as Death's identity. Lilia, who experienced the end of her life non-linearly, was able to tell the coven this and have Agatha confirm that she already knew. Death realized that Billy was reincarnated into William Kaplan's dead body and responsible for the Witches' Road hex dimension, and approached Agatha alone to confront her about this affront to death; Agatha demanded Death leave her alone, including after her death, in exchange for Agatha delivering Billy to her. Death reluctantly agreed and cut her way out of the Witches' Road.

Agatha helped Billy leave the Witches' Road without surrender to Death; when Agatha then left and returned to her garden in Westview, Death confronted Agatha over this betrayal and Agatha's general rejection of her. Billy granted some of his power to Agatha so she could fight Death, and Death flung Billy away. Agatha soon realized the futility of fighting Death, being pinned back; Death demanded one of their souls and Agatha ultimately chose to sacrifice herself, taking Death by surprise. Agatha embraced and kissed Death, which killed her. Death shed the appearance of Rio to reveal her true form to Billy as she told him to go.