The mad Titan, Thanos.
The MCU has been building up to a showdown with this badass for a while but who, or what, is he?
In order to answer that question, and more, I need to give you a history lesson. Marvel style.
The celestials are a race of god like aliens. Ancient, powerful and man do they just fucking love meddling with shit. For reasons unknown (at least to my knowledge) they have tampered with the evolution of a number of races. Most notably the Skrulls, the Kree and us, the humans. When the Celestials came to earth, they found a primitive ape species which they used as a base to create two, new and distinct, species. Homo Immortalis, also known as the eternals, and Homo descendus, the Deviants.
The deviants gained mutations, in an evolutionary sense, much faster than humans. This trait accelerated their culture centuries past their primitive human rivals (us) by 20,000 BCE they had advanced technologies such as genetic engineering, and great cities. If it wasn't for the infighting, and the endless war with the eternals, the deviants would have conquered the whole planet.
When the Celestials returned to earth, the deviants attacked their spaceship and were almost annihilated by the celestial counter attack.
The Eternals of Earth.
Under the leadership of Kronos (not the titan of greek mythology, although that guy exists too) the Eternals created one of the most advanced cities in the history of the Earth, Titanos. Kronos wanted peace, and to understand their place in the universe, where as his brother, Uranos, wanted to conquer all beings he considered beneath them (pretty much everyone) the difference of opinion led to a civil war.
Uranos and his followers were defeated, and banished from the earth.
Now back to Thanos
The banished Eternals traveled deep into our solar system and found an abandoned Kree outpost on one of Saturn's moons, Titan. They took up residence here. Thanos was born on Titan. Yeah, Thanos is technically human.
He was born an Eternal, only he has the deviant gene. When he was born his mother had a vision of the death he would cause and immediately tried to fucking murder him. Yup. He is obsessed with death, whom he met once while it was in the form of a woman. His plan is to murder enough shit to win her heart.
In one story, after he collects enough of the infinity gems for his Bling glove, he straight up kills half of everything in the galaxy. Yup, half.
So, yeah. The mad Titan, as in...he's from Titan and he's mad.