In the MCU, Thanos was an appeasing despot who felt our planet's huge population of people would eventually exhaust all its sources and die out. His objective was to prevent instability in the galaxy by ensuring consummation of life for every single living thing.
Thanos searched for the Infinity Stones in order to match their power so that he could complete this task.
For a long time, the Eternals and Deviants have been intrinsically bound to each other. Early proto-humans were the biological objects of experiments carried out by Celestials, and they later produced three new genetic branches. This consisted of the Deviants, followed by Eternals, and a particular branch of baseline humans that housed the genetic enhancement seeds that ended up coming to the Earth event.
After deployment to various celestial worlds, the Eternals were deployed to safeguard the evolutionary progress of sentient beings and to give power to the Celestial seed residing inside the planet that absorbed the energy produced by the life forms. Nine of these Eternals attended a meeting that a blog post made.
The Celestials carefully bred the Deviants to ensure the steady emergence of sapient life by protecting the nascent Celestials from the planet's top predators.
Thanos' parents were two Eternals who believed that he was too dangerous for the Eternal species to be with, which led to the thought that he was a dangerous Deviant, when the Deviant gene was found in him. Thanos is an Eternal, but he is an unusual version of an Eternal since he harbors the Deviant gene within him, making him a highly uncommon member of his people.
All together, mad Titan is a mixture of both Eternal and Deviant.