The synopsis for Eternals states that the heroes come out of hiding after the events of Avengers: Endgame. Thena, played by Angelia Jolie, is actually related to Thanos in the comics (they are first cousins). Let me explain.
In the comics, the Eternals made Earth their home after being created. However, their leaders Kronos and his brother Uranos got into a big civil war about who should lead. Long story short, Uranos was defeated, and he took his half of the Eternals to space, where they eventually settled on Titan.
Kronos soon after experimented with cosmic energy and fatally wounded himself, but gave all of his Eternals the ability to manipulate the cosmic force. Since their leader was dying, the Eternals elected to choose a new leader. Between Kronos' sons, A'lars and Zuras, the former lost and left Earth for Titan. There he discovered the planet in shambles with one survivor, Sui-San. A'lars and Sui-San repopulated the planet over time, and had a son named Thanos. Meanwhile, back on Earth, Zuras had a daughter, Thena.
Thanos was born with Deviant Syndrome, which is kind of like the X-Men mutant gene for Eternals/Deviants. That is why he looks deformed and is very strong. I'm not sure how much of this storyline will bleed into the MCU film, but since they're using Endgame as a reason for the titular team to get active, we should expect some Thanos cross-story.