Alex and Fire Emperor talked for a short period of time and got familiar with each other.
When Alex heard from Fire Emperor about the three Demon Monarchs, he was shocked. He knew he has to do something otherwise, if the human race is destroyed, his mission would fail. He is so close to completing the mission, he can't afford to fail right now.
Thus, he proposed to Fire Emperor that he was going to take care of the Demon Monarchs. However, Fire Emperor stopped him. He told him to guard the mouth of Abyss and not let any demon out.
As for the three Demon Monarchs, he was going to deal with them. They are his hunts!
After talking with Alex for some time, he already knew that Alex has reached the Extreme Celestial Phenomenon Stage. So he offered Alex to stay here and guard the mouth of the abyss and restrict any demons from coming out.
After dealing with the three Demon Monarchs, he would return. At that time, he could go back to the Inferno Kingdom.