Arishem is a member of the Celestial race. On Earth-9997 the Celestials have no physical bodies and have evolved to the point where they have almost lost all definition. In order to maintain any definition they had left, they created armors out of Vibranium. Arishem and the other Celestials were responsible for the collapse of the previous universe, as the Celestials required to implant embryos into the cores of planets in order to reproduce.
Since the creation of the current universe, the Celestials continued their procreation goals, however they have opposition in the form of Galactus, who was created as a counter balance to the Celestials in order to prevent them from over populating the universe again.
Arishem's history mirrors that of his Earth-616 counterpart, at least in terms of his activities in recent history. When the entire human race was mutated 200 years before their time, Arishem and his fellow Celestials returned to Earth to pass final judgment. They were met with opposition from Black Bolt on the Moon, Arishem and the Celestials would kill Black Bolt and then set their sights on Earth.
On Earth, they would be opposed by Namor and an army of sea creatures, Tony Stark in a giant robotic factor and an army of Iron Men robots, and an army of Asgardians led by Loki. This bought the Earth precious moments for the arrival of Galactus who was called to Earth by Black Bolt before his death. Galactus was able to fight off the Celestials, destroying some and forcing the others into retreat. Arishem's current whereabouts are unknown.