The Arbiter is one of a number of beings that can evolve when a Stardust infection becomes serious.
The path of evolution that an infected individual will take depends largely on its physical condition, as well as the environment.
The morphology of the Arbiter is a good example of this. It has developed one ultra-strong arm which it swings to attack, as well as a strong torso to support the arm's increased weight.
Arbiters have been known to break through weaker defensive structures with a single swing of that giant muscular arm.
