It turns out the Moon Presence is actually just the Angel of Death sent to deal with the Egyptians.
He was redeployed in another universe to deal with Yharnum, but they kinda just rolled with the rivers of blood and unending night. They even shook off the death of the firstborn pretty well...except for Oedon; he kinda went berserk when he lost Mergot.
MP was about ready to give it up and turn this place into the next Gomorrah, when Anderson showed up.
Now, he's offering the good priest a job--killing multiversal heathens that are too bizarre to be bothered by Plagues.