The Reaper.

The two of them are level 5 demon kings and they had to struggle to kill level 4 Celestials in a one-on-one fight. Meanwhile, Aeternus is currently fighting 3 Celestials at the same time after killing 4 other Celestials at once. One of those four Celestials also includes the Celestial Supreme of this plane. 

The proof that Aeternus is not what he seems to be is right there in their face but the snake demon king can't see it. It is either due to plain ignorance, pride, or blind confidence. Either way, Aeternus is very weird. This black energy that he produced is very weird.

'I just hope this cooperation will end on a good note. Then maybe I can celebrate by fucking the brains out of this dunce.' She thought to herself. Then she licked her lips while glancing at the snake demon king.