After being set up in various films throughout the course of the Marvel Cinematic Universe, Thanos made his existence-decimating debut in Avengers: Infinity War. The movie revealed his goals to "save" the universe by wiping out the population, as well as his reasoning.
Thanos himself reveals he's plagued by the loss of his home world known as Titan, which ignored his warnings and his call to cull the population. But it doesn't reveal how Thanos escaped the fate that seemingly befell the rest of his people.
According to Avengers: Infinity War — The Art of the Movie, Thanos was exiled from Titan after his proposition, so he was not present for the planet's destruction. It's a tragic accident, and one that drives Thanos' actions to obtain the Infinity Stones.
The Mad Titan has been the most deadly and powerful villain yet in the Marvel Cinematic Universe, and after being teased in multiple movies over the years the character had to have a major payoff.
“Thanos has been lurking in the shadows with a desire to obtain these Infinity Stones, which has played a big part in our other films,” Feige said in Marvel Studios: The First Ten Years. “We’ve introduced the Tesseract, revealed to be the Space Stone; the Mind Stone, which came out of Loki’s scepter and then went into Vision’s forehead; and the Time Stone: the Eye of Agamotto that Doctor Strange wears is an Infinity Stone itself! And of course the Guardians dealt with the Power Stone.
"So these storytelling devices that we’ve seeded into every film will continue to play a part and come together. We’ve been teasing this for six years. That’s a long time to tease something cinematically before paying it off. Thanos has to be the greatest villain in our movies.”
And now that Thanos acquired all the stones, he's basically unstoppable. So how could Earth's Mightiest Heroes stand a chance to defeat him in Avengers 4? The film's co-writer Christopher Markus offered a slight tease in the art book.
"They've been misused by people who didn't understand their true power. And now that the real master of them, Thanos, has shown up we can see what they actually do," Markus said. "The problem is, as you can imagine, as you gain control over these things, your story just goes haywire because why are there ay stakes for the man who can control time, space, reality, your mind, your soul and power? He can reset as many times as it'll take. So you have to build in a lot of bumps in the road or maybe keep one stone away from him for long enough in order for the thing to have any dramatic structure at all."
We'll see how the heroes stop Thanos in Avengers 4, premiering in theaters on May 3, 2019.