Kings Of Trash Talk.

So even if they are willing to sacrifice all of their energy to kill Aeternus, they have a limit on their speed. He has that same speed, and for some strange, unknown reason, he never runs out of energy.

So they can't even overcome him with perseverance. They have always been the ones to run out of energy whenever they chase him. Meanwhile, he always gets stronger after each battle. 

All of these have made them fail to kill him several times, but their numerous attempts have only enabled him to grow stronger. This has further increased the threshold of energy they need to sacrifice to threaten him. Not that that matters anymore. 

All of their failures are why he has grown to reach a height of 10 kilometers. He is actually bigger than this. He is ten times bigger than this, to be exact. But he can shrink himself, unlike other demon gods.