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The Outriders are supporting antagonists in the 2018 Marvel blockbuster film Avengers: Infinity War, as well as in its 2019 sequel Avengers: Endgame. They are an animalistic species of aliens that are subservient to the infamous Thanos, the Black Order and their sister species known as the Chitauri.
Avengers: Infinity War
In his quest to collect the six Infinity Stones, Thanos collected the Power Stone (the Orb) by destroying the planet Xandar, and the Space Stone (the Tesseract) by slaughtering all of the Asgardians (including Loki). After collecting the Reality Stone (the Aether) from the Collector and the Soul Stone from the Red Skull, Thanos sends the Black Order to Earth to collect the Time Stone (the Eye of Agamotto) and the Mind Stone.
Though Ebony Maw captures Doctor Strange (the current holder of the Eye) and delivers him to the planet Titan for Thanos, Corvus Glaive and Proxima Midnight failed to steal the Mind Stone from its current holder (the superhero Vision). Even when Maw ends up being killed by an arriving Iron Man, Thanos collects the Time Stone after defeating several heroes. With victory at hand, he orders Glaive, Cull Obsidian & Midnight to lead an Outrider attack on the country Wakanda, knowing that Vision is currently residing there trying to remove the Mind Stone and destroy it.
To that end, the Black Order summons several giant ships containing countless Outriders to the scene, even when the Wakandan warriors and several Avengers activated their vibranium shields to hold them, several Outriders were able to go through shields without even dying. This forces the Wakandan warriors and the Avengers to fight back against the Black Order and the Outriders in order to buy enough time for Shuri to remove the Mind Stone from Vision.
At first, the heroes are being overwhelmed as they are too many Outriders to fight back. Fortunately, Thor arrives to the rescue, bringing in two Guardians of the Galaxy (Rocket Raccoon and Groot) and a new powerful weapon called Stormbreaker, allowing the heroes to finally defeat the Outriders and kill them all as well as the Black Order, allowing enough time for the Scarlet Witch to destroy the Mind Stone.
Unfortunately, despite the loss of the Black Order and the Outriders, Thanos arrives and foils this by defeating the Avengers and Guardians and using the Time Stone to reverse the event, allowing himself to collect the Mind Stone while killing Vision at the same time. Even when Thor managed to use Stormbreaker to violently stab Thanos in his chest, Thanos used the power of the Stones to wipe out half of the universe's populace (including several Wakandan warriors, Avengers, and Guardians) in retaliation, taunting Thor that he should have aimed for the head instead. As a result, the surviving heroes mope over their failure while a triumphant Thanos escapes and oversees a sunrise from another planet.
Five years after Thanos' destruction of half of all life, the remaining Avengers and Guardians use the properties of the Quantum Realm to travel back to different points in time to collect the Infinity Stones and use them to undo Thanos' actions; however, due to the sensors of Nebula's past and future selves interfering, the Thanos of 2014 ends up learning of the Avenger's plans and that they had killed him in their time. Knowing this, Thanos decides to use the Stones to wipe out all life and create a new universe, and has 2014 Nebula impersonate her future counterpart and bring his ship and all of his forces, the Outriders included, to the future.
After bombing the Avenger's headquarters and defeating Thor, Iron Man and Captain America, Thanos unleashes the Outriders and his other minions upon the earth to destroy the Avengers and ravage the planet. However, they are opposed by an army of the Avenger's revived allies, who fight with the Outriders and the others as they try to get their hands of the Infinity Gauntlet. However, in the end, all of the Outriders, along with Thanos' other forces and Thanos himself, are killed when Iron Man sacrifices himself in using the Infinity Gauntlet to destroy them all at once.