The Calamity Of Demon Kings.

A demon king is the equal of a Celestial and there are 10 Celestials in this divine plane. He can't take all of them on. At least, someone of his apparent strength should not be able to take on all of the Celestials. It is expected that he will die several times. While he can return after death, he will just be wasting his Authority in resurrecting so it is better to ask for help. Which he did. 

These two accepted his request despite his fearsome reputation for killing demon kings. They are strong and they believe that they will be able to protect themselves from him. After all, if he was so strong, he should be able to invade the plane alone and not share the loot from the divine plane.