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Spider-Man: So, what is this guy’s problem, Mr. Stark? Iron Man: He's from space, and he came here to steal a necklace from a wizard.
~ Spider-Man and Iron Man talking about Cull Obsidian.
Cull Obsidian is a supporting antagonist in the Infinity Saga of the Marvel Cinematic Universe, serving as one of the secondary antagonists of the 2018 Marvel blockbuster film Avengers: Infinity War, and as a minor antagonist in its 2019 sequel Avengers: Endgame.
Obsidian was first seen raiding the Asgardians' ship following the destruction of Asgard by the deceased Surtur. Along with his father and siblings, he slaughtered the Asgardians to their deaths just as Thanos fights against the raging Hulk. Although Obsidian intends to help Thanos, Maw tells him to stand down, knowing that Thanos is too strong for the Hulk to defeat. When the defeated Hulk is sent back to Earth by Hemidall (who ends up being killed by Thanos), Obsidian helps his siblings in forcing Loki in surrendering the Space Stone (the Tesseract) to Thanos. It is also revealed that Obsidian was involved of his father's attack on the planet Xandar to gain possession of the Power Stone (the Orb), which he used to destroy Xandar. After Thanos kills Loki, Obsidian escaped with his family as Thanos uses the Orb to destroy the Asgardians' ship, leaving a helpless Thor to drift in space.
With two of the Infinity Stones in place, Thanos sends the Black Order to Earth to collect the Time Stone (the Eye of Agamotto) and the Mind Stone while he personally travels to Knowhere to collect the Reality Stone (the Aether) from the Collector and Vormir to collect the Soul Stone from the Red Skull. Assigned to collect the Eye of Agamotto, Maw and Obsidian tracked down its current location in New York City, where they laid down an attack in to draw Iron Man, Spider-Man and Doctor Strange (the current holder of the Eye) into the scene. Despite the heroes' efforts, Maw managed to capture Strange and flies away on his ship, intending to deliver the Eye to Thanos at the planet Titan while Obsidian meets up with Proxima and Glaive after their failed attempt to collect the Mind Stone.
After collecting the Aether, the Mind Stone and the Eye of Agamotto, Thanos sends Proxima, Glaive and Obsidian to Wakanda, ordering them to lead a massive army of Outriders to attack the country and obtain the Mind Stone at all costs. This forces several Avengers and Wakandan warriors to fight back against the massive army. During the battle, Obsidian engaged into a fight against Bruce Banner wearing a Hulkbuster suit. Though he managed to tear off one of Banner's arms, Banner set the arm flying by activating its thrusters, sending Obsidian into the forcefield protecting Wakanda until he exploded to his death. His siblings and the Outriders would later meet the same fate after being killed by the Avengers and Guardians.
Despite the losses, an arriving Thanos didn't care about Obsidian's death and managed to collect the Mind Stone after killing Vision. Thanos then used all of the Infinity Stones to wipe out half of the universe's populace (including several Avengers and Guardians), much to the heroes' distraught.
He can be seen during the final battle, along with the rest of the Black Orders. Cull began to fight Korg before being stepped on and subsequently killed by Giant Ant-Man.
In the comics, Obsidian was the named Black Dwarf. Interesting enough, in the comics Cull Obsidian was another name for the Black Order. This was changed to not confuse the names Black Panther, Black Widow, and Black Order, with Black Dwarf, despite the group being renamed "Children of Thanos".
In concept art and according to the creators of the movie, Obsidian was originally supposed to have a different uniform, and is confirmed to be made up of pieces of his victims and those he defeated, including original plans to wear a belt of skulls.