All Thanos does is to win Lady death’s affection. For long, Thanos has remained in love with the physical embodiment of Death in the Marvel Comic Book Universe. While that infatuation is not shown in the MCU, the comics tell a different story. Thanos kills and maims in Death’s honor. All he wants is for his deadly mistress to pay attention to him.
One time, Thanos got more than he bargained for. Lady Death & the Mad Titan hooked up. The result was an abstract and nigh omnipotent entity called the Rot. And thus began the downfall of the Marvel Comic Book Universe.
The Rot started out as a void of literal nothingness. it existed and didn’t exist at the same time. But it soon began to grow. And after developing its reach beyond anything Death and Thanos could have ever imagined, the Rot threatened to end the Marvel Comic Book Universe.
The Celestial Quest series sees Vision and Mantis trying to retrieve a captured Sequoia. Sequoia is currently imprisoned within the Rot’s dark void. Mantis and Vision try to enlist Eternity’s help in stopping Thanos and the Rot. Thanos intends to kill the Celestial Messiah. Eternity refuses to partake in the superheroes’ rescue mission, saying whatever is going to happen, will happen. The Rot’s powers grow by leaps and bounds. It ends up killing Thanos’ father and its grandfather Mentor as well many others that oppose its power.
The series finale sees Lady Death coming to Thanos. The Avengers are also there, trying to fight the Rot. The dark entity had already crept into the Avengers’ ship, killing a servant of Thanos in the process. The Avengers go out into the Rot, trying to attack it head on. Scarlet Witch claims she hasn’t sensed a “magick so unutterably dark”. Thor’s Mjolnir has no effect on the Rot.
Death says the Rot is her and Thanos’ child. the Mad Titan is confused since he never got intimate with her. It is then Death explains how the Rot came into being. It was the result of Thanos’ energy bonding with Lady Death’s in a previous encounter. The Rot started out as a mere dot, its presence so minute no one could feel it. But now the monstrosity stands to corrupt the entire universe.
Thanos and Death manage to kill the Rot and put it out of its misery. Death finally agrees to accepting Thanos as her eternal mate. Thanos rejects her hand, stating: “if you will have me, you are not worthy.”
The Rot was alive but didn’t have sentience. It is indeed one of the greatest forces that ever came into being. Thankfully, the Avengers never need to face it in the MCU.