Ken Connell is the original wielder of the Star Brand. He gets Debbie the Duck pregnant. Duck tells him its his baby, but he doesn't believe her. She ends up having it amid chaos after Ken accidentally destroys Pittsburgh, with the help of Roger and Jane Price. Duck dies in the process.
This baby is the Starchild, a baby with the innate power of the Star Brand. The Starchild absorbs Duck's memories and goes off to learn more about life.In fact, it was already learning even while in utero: it tells Roger and Jane that its mind was exploring the solar system while it was still a fetus.
It has several mishaps and kills a variety of people without understanding the importance of such a thing. It goes back to Roger and Jane to learn from them, but kills Roger. Then it goes off into space to confront Ken, who has gone mad. It removes his Star Brand power, and realizing its error with Roger, resurrects him.
It tries to do good in the world, removing all death, but is eventually convinced to reverse this decision. It also deactivated all the nuclear weapons on Earth.
It eventually realizes the Star Brand is too much power for anyone. It brings all the people (except Jim Hanrahan) who wielded the Star Brand to Jupiter. It turns out that the Starchild, Ken, and Ken's opponent the Old Man are actually all the same being. It merges them all together, then manipulates reality to fix their temporal loop. Ken's essence is sent back in time to become the Old Man, the Old Man's essence is sent briefly back in time to become the Starchild at the moment of its birth, and the Starchild becomes Ken before he gained the Star Brand.
This fixes their temporal paradox, but doesn't quite seal away the Star Brand as intended. Hanrahan keeps his Star Brand, and Quasar eventually meets him and brings its power back to Earth-616.
The Starchild had the full use of the Star Brand, unlike his father Ken. As such, it had seemingly near-infinite powers. It was capable of massive energy blasts, immortality, immense durability, telepathy, flight, the ability to resurrect the dead, the ability to stop death over the entire Earth, cosmic awareness, temporal manipulation, and extensive matter manipulation and reality warping. Even while it was a fetus, it was capable of intentionally projecting its mind throughout the solar system.