Marvel's Thanos has long sought the affection of the living embodiment of Death, but when she revealed they actually had a child who had become out of control, he decided to put it out of its misery. In the Celestial Quest arc, the Mad Titan and Death created a black void called Rot, which started out as literal nothingness and turned into a threat strong enough to fight both Thanos and the Avengers.
In the Celestial Quest series from Steve Englehart, Jorge Santamaria, and Scott Hanna, Mantis and Vision seek out Sequoia, who's been captured and taken to a void called the Rot. Mantis tries to get Eternity to help stop Thanos, who intends on destroying the Celestial Messiah, but their pleas are rejected. Matins warns the cosmic entity about a dark cancer that will consume him, but he accepts that whatever will happen, will happen. As the story continues, the Rot's growing presence kills Thanos' father Mentor, and those who stand in its way.
In the series' final issue, Death reveals herself to Thanos and the Avengers. As she appears, the Rot slowly begins to make its way into the Avengers' ship and instantly disintegrates one of Thanos' servants. Mantis and Sequoia manage to stop the Rot temporarily as the Avengers go out in space to take it on directly. Scarlet Witch notes she's never felt a "magick so unutterably dark!" When Thor throws Mjolnir at the Rot, it just bounces back at him. Each of the heroes tries their most powerful blasts, but they do nothing.
Back on the ship, Death tells Thanos the Rot is their child. The Mad Titan is confused, as he's never been intimate with Death, but she informs him that when he had previously died, their energy bonded and created the "horror." The Rot started out as a small dot in the universe that Death didn't even detect, but slowly group larger and more terrifying. Death states the Rot is alive, but doesn't have life. She tells Thanos the only way to stop it is to do it together.
Thanos and Death head into the Rot and slowly are corrupted by it. The pair use all of their power to dissolve their offspring, resulting in the universe screaming after the mammoth blast erases Thanos and Death's child. Following the Rot's demise, Death seeks Thanos' hand and accepts him as her mate. Thanos rejects Death and says "if you will have me, you are not worthy," before leaving.
While the Rot might not have a living, physical form like Thanos' son Thane, it used the energy from Thanos and Death to become a cosmic nightmare that destroyed everything it consumed. The Avengers stood no chance against the Rot, as it could only be defeated by its parents. In the end, Thanos and Death worked together to ensure the Rot couldn't swallow the entire universe. For a one-and-done character in Marvel Comics, there are few more powerful than Thanos' forgotten child, the Rot.