As the survivor of a dead planet hellbent on wiping out half of all life in existence, many Marvel fans are wondering how Thanos grew to be so powerful in Avengers: Infinity War. While the Infinity Stones go a long way to explain his abilities, it doesn't reveal how Thanos controls both the Chitauri and the Outriders.
But Marvel published a novel titled Thanos: Titan Consumed (via HNEntertainment) which goes into detail about the Mad Titan's rise to power, revealing just how Thanos came to lead two separate groups of warriors.
In the novel, Thanos left Titan and fought his way through the galaxy, attempting to get to Asgard but instead happens across the Chitauri homeworld. According to the novel, the Chitauri are a hive mind, which was hinted at in Marvel's The Avengers when their portal collapses, resulting in many of the aliens dropping lifeless.
In the Chitauri's first appearance in the comics, they were shapeshifting aliens that were considered stand-ins for the Skrulls in The Ultimates, attempting to infiltrate various positions of power on Earth. But in the mainstream Marvel Universe, their appearance and hive mind physiology was more akin to how they are in the MCU.
But in Titan Consumed, Thanos comes across the Chitauri leader known as the Other, who allows the Mad Titan to assume command of the hive mind and lead them to war in their attempts to conquer the galaxy.
Because of the unruly nature of the Chitauri hive mind, Thanos desired a more controllable, autonomous fighting force and began experimenting on his own genetic creations. Using his own DNA as well as that of the Chitauri, the Mad Titan engineered a new race of creatures that he dubbed the Outriders.
Fans caught a glimpse of Thanos' utilizing Chitauri forces in the flashback sequence from Avengers: Infinity War, when they invade Gamora's home world to split the population and execute half of them. But by the end of the film, the Mad Titan instead utilizes the Outriders in his invasion of Wakanda.
Marvel Studios has since shed doubt on whether or not this story is canon recognized in the cinematic universe, but it's still taking some obvious cues from the events of the movies.
Maybe we'll learn more about Thanos' backstory when Avengers 4 premieres in theaters on May 3, 2019.