Rot is a malevolent cosmic entity native to the Marvel Universe and is the offspring of Thanos and Lady Death - this makes it a dangerous anomaly that was a threat to all things, both living and deceased.
It's the main antagonist of Avengers: Celestial Quest by Steve Englehart.
Death had always kept a distance between herself and Thanos but when Thanos died, turned into stone by Adam Warlock, all his energy merged with Death’s, and as a result, the Rot was created. The offspring of Thanos and Death was quite small and minuscule so Death did not notice it. Although it was a moving void of nothingness it slowly consumed more, and grew. When Death finally noticed it, she realized she had not noticed it before since it was alien to her (it was an abstract being with no presence) and she held no sway over it. Raptra, who was flying with Quoi in a ship, told Quoi they were flying into the Rot. When Quoi asked what it was, Raptra said that the Rot was a patch of space that a pirate can hide in. When Quoi noticed a blackened area, Raptra told him that it was the Rot.
Reptyl Prime and Primo received information that Raptra was heading toward the Rot. When they asked about the Rot to their informant, he said it just popped up one day. He also said going inside it was suicide, but it was good for hiding. Meanwhile, the Avengers spotted the hole in space (the Rot) everyone was headed toward, and Vision and Celestial Mantis left to seek Eternity for aid. When they met him, Mantis told Eternity that the Rot was like a cancer to him, and that it would eat and kill him. To her surprise Eternity told her that he couldn’t affect the Rot, after which he then dismissed the heroes. Meanwhile Rot found and killed Mentor.
When they returned the Rot swallowed them both and others who were found in the zone like Thanos and his minions. The Avengers entered the Rot to battle Thanos (later revealed to be actually one of the Thanosi) and his minions and rescue Mantis and Vision, who were all in the Rot as well. The Rot had grown since it was last seen. It appeared that the Rot was following all living things, as if it was sentient. After a fight between Thanos and his minions and the Avengers, Thanos finds he cannot teleport, and that Rot had completely encased them.
Mantis made a bubble to protect the ship from the Rot. But the Rot broke the bubble and killed Primo with one touch. Mantis and Quoi then make a bubble together, which managed to resist the Rot. Thor, the Scarlet Which, Reptyl and Vision all then went outside the ship under the protection of the bubble, and channeled their powers into Rot to stop it. Rot was completely unaffected by their attacks. Mantis managed to hold the bubble by herself as Quoi decided to go against Rot. He thought that his powers may affect it since they were unconventional and he was not human. Quoi then enters the Rot and finds he couldn’t absorb it since it was too big. He then tried to reverse its power, but the Rot enjoyed it. Quoi returned to the ship and Death suddenly appears to reveal the Rot's origin. Thanos and Death then go out to destroy the Rot, using a plan Death concocted. Death and Thanos discover that neither of them can affect the Rot directly, but the Rot was their essence. So they both went against their nature. They released their power and Death pushed as Thanos pulled, and the Rot was dissolved. After that all those killed by the Rot were restored.
Powers and Abilities
As its name suggests the Rot has the power to destroy anything it touches. Whether it is matter or energy, anything that comes into contact with the rot will rot and disintegrate almost immediately. It is virtually indestructible as it cannot be destroyed by any physical energy attacks and it cannot be harmed or injured. It is of an enormous size of emptiness which continues to expand, taking up whole portions of space. Another of its abilities is that it is able to absorb energy directed against and add to itself. The rot is considered both living and dead, since it is the offspring of Death and Thanos, meaning that nothing living or dead beings could effectively harm it. It has proven so powerful that neither Death nor even Eternity can affect it. Of everything the Rot is capable to destroy it has no apparent affect on inorganic matter, such as Thor's hammer.