Warning! Spoilers for Eternals: Thanos Rises #1 by Marvel Comics below
Marvel Comics has taken a deeper dive into the events that helped create Thanos, revealing the dark fate of his parents Mentor and Sui-San. In Eternals: Thanos Rises #1, it's revealed the two Eternals are put into exclusion after the creation of Thanos as Mentor is forced to watch all of the murders his son commits while trapped inside an inescapable room.
Thanos' parents have been mysterious absent in the newest run of the Eternals. Famously, his mother Sui-San attempted to kill him after giving birth and seeing he was born with the Deviant gene. Meanwhile, Mentor also tried to destroy Thanos later in his life, creating Drax the Destroyer to take down his monstrous son after he conquered their homeworld of Titan. Now, Marvel Comics is revealing what ultimately happened to Sui-San and Mentor, and the answer isn't particularly pretty, as the Eternals made sure to lock them away as an attempt at peace backfired in the biggest way imaginable.
In Eternals: Thanos Rises #1 by Kieron Gillen, Dustin Weaver, Matthew Wilson, and Esad Ribic, the story of how Thanos came to be is revealed. In the past, Mentor strikes a deal with his mother Daina to reset the current warring Eternals' minds and is given a lifetime to see whether the ancient race can have children - as the Eternals are created, not conceived. His attempts prove successful, as he meets Sui-San, a warrior who fights alongside the Uranites. The pair fall in love, but even after a warning from his father Kronos, Mentor is given Quantum Bands, allowing them impossible to be possible. However, conceiving Eros, Sui-San gives birth to another child later, Thanos. The villain ultimately kills both of his parents
Upon reawakening in the Great Machine, Mentor tells the Eternals his attempts have failed and the race should not breed. He's thrown into exclusion in a cell next to his wife, who wants nothing to do with him. Inside exclusion, Mentor is notified every time Thanos killed someone, showing his attempts at creation led to the unimaginable death and sorrow. Mentor and Sui-San waste away in exclusion, as the only way they can be freed is if Thanos destroyed the Earth - which would still be bad news for them, as he'd try to kill them after doing so.
Mentor and Sui-San being locked away in exclusion explains their absence in the current Marvel Universe. The Eternals were worried the Mad Titan's birthparents would create another Thanos if they were given the chance, so they imprisoned them to avoid a second monster being born. For the sake of Earth and the Eternals race, they need to pray Thanos never finds out about their true whereabouts.