Despite Thanos' infatuation for Death, she had always kept his distance from him and Thanos was certain that he had never been intimate with her. However after his death in the Final Threat story arc, Thanos was finally united with his love, and in Death's realm their energies merged. It was out of this union that Rot, an entity, half of abstract heritage, and half of eternal heritage, was born.
Rot was created by Steve Englehart and Jorge Santamaria and first appeared in Celestial Quest #2. He wasn't properly introduced into #5 however.
Major Story Arcs
At first, Death was unaware of her child's presence- referring to him as a speck. But his power increased to the point that when Thanos was tasked by Death to kill Rot. Thanos not only gleefully agreed, but was disgusted that he had actually created life, and annoyed that he didn't remember being intimate with Death.
Soon after, Thanos killed Rot with a blast so powerful, it was said to make the universe scream.
Powers and Abilities
Being the son of Abstract entity and Skyfather-level being, Rot probably had the capacity to develop vast powers on the level of his parents, including Matter and Reality altering powers, enhanced physical abilities, blasts and other powers far above the scope of most super-humans. He seemed to have no fixed form, and almost acted like a force of nature. He was slowly eating away at the universe, destroying it, and leaving various Avengers catatonic just by touching them. At the time of his death, his power was already such that his mother tasked his father to kill him, and the very scope of the blast Thanos used to dispatch his son indicates that he was a valid threat. Rot, being the son of the Embodiment of Death and the Avatar of Death, would have almost certainly been immortal if it wasn't for his untimely demise. However, the power of his parents means its possible he may return one day.