At the beginning of time, when Eternity first manifested itself, Entropy was created. Entropy is an avatar of Eternity, and his entire purpose is to destroy, making him Eternity's opposite.
When first seen, it was revealed that Entropy caused Captain Marvel's cosmic awareness to go out of control, driving him insane. When Genis refused to change places with Marvel, leaving him trapped in the Microverse, Rick Jones sought out a spiritual healer and prophet called Shingo Doon to help him in ridding Captain Marvel of his insanity. Before Shingo Doon could help him, he was destroyed by Entropy, who's voice then called to Rick and told him there was nothing he could do and that Marvel's fate was sealed.
For 28 days following this event, Rick tried to escape the mountain cabin in which Shingo Doon had lived. Every time, Entropy stopped him by causing an earthquake or an avalanche.
Escaping the Mountain with no prior knowledge of how, Rick awoke in a room with a woman called Epiphany. Investigating the unknown surroundings, he heard Epiphany talking to Entropy in the bathroom. Wrapping himself in a blanket, he went to confront Entropy. As with most cosmic beings, Rick saw a cosmic looking figure facing him wrapped in an identical blanket, mirroring his own stance and visage. Entropy revealed that its appearance had mirrored Rick's since Entropy normally only appeared to people in a form they could understand. Around that time Captain Marvel tried to kill himself, and due to their symbiotic relationship, Rick collapsed on the floor.
Entropy met with Captain Marvel and revealed to him that he was the one that had made Marvel go insane. Entropy showed Marvel a series of different situations in which a man tried to please his father. He then went on to reveal that he was the offspring of Eternity, and that he was out to destroy the universe. He wanted Marvel's help, which was why he had driven him insane. Marvel gladly accepted.
Entropy, Epiphany, Marvel, and Rick stood in pure, white, nothing. Entropy and Marvel had apparently been successful in destroying the universe and they had killed Eternity. After an argument with Rick over whether or not Marvel was God, Marvel asked Entropy on that fact. Entropy didn't answer. Entropy said that he had achieved the purpose for which he was created. Now that he had destroyed everything in the universe, there was nothing left for him, he was pointless and didn't know what he would do next or what he could do.
It was Rick who suggested he create something. Creating something was the first new idea Entropy had ever had and decided to do it. However, he had never thought of or experienced creation and didn't know how to begin. Rick told him just to go with what he knew. Given that all he knew was what and how his father was, he created a big bang, recreating the entire universe, and became the new Eternity. Everything returned to normal, excluding the fact that Genis was still insane.