Spoilers for Eternals #2 below!
In the Marvel Comics Universe, killing Thanos is next to impossible. It's not because the Mad Titan is incapable of being defeated in battle, but instead, it's because, at times, he's not allowed to die. In the latest issue of Eternals, readers are reminded that Thanos is effectively immortal and it's all because Death won't allow him to enter her realm.
Thanos has long sought the affection of the living embodiment of Death, as he wiped out half of existence with the Infinity Gauntlet in an effort to impress her. However, Death has never accepted Thanos' love as he had hoped. Instead, she's spurned his feelings - so much so, that in Cosmic Powers #1, she banned him from her realm, effectively making the villain immortal. That's on top of all of his original Eternal-related powers, including immunity to all disease and non-aging. While his ban and immortality were thought to be removed, in this week's Eternals, it seems that might not be the case after all.
In Eternals #2 by Kieron Gillen, Esad Ribic, Matthew Wilson, and Clayton Cowles, Ikaris and Thanos battle across space and time in a fight on Titanos. The Eternals outcasted Thanos, an Eternal born with the Deviant gene giving him his monstrous appearance, and looked down on him even more thanks to the Snap. The two powerful Eternals exchange blows, as they travel through portals and different time periods. However, after Ikaris does damage to Thanos, it's noted that the Mad Titan can't be harmed so easily, as his wounds close as "Death itself rejects him from her embrace."
Since the fight lasts both seconds and for all of time and history, it's unclear whether Thanos is affected by the previous ban from Death's realm because of time-travel shenanigans, or if it's currently active. It wouldn't be surprising if it was the latter, as Thanos has died multiple times in recent Marvel Comics history, only to continually come back. If Death is preventing him from truly dying then that would explain his returns. You'd think that the Infinity Stones might play a factor in why Thanos can't be killed for good, but instead, it's a curse from the being whose affection he seeks the most.
Thanos is back and wants to kill the Eternals for good. With the Machine that revives Eternals now inoperative, the Mad Titan holds a key advantage to those who have previously outcast him - he seemingly can't die thanks to Death. The Eternals, meanwhile, have to figure out how to stop Thanos while losing their immortality. It seems like quite a tall task for the ancient beings.