One god under two names, across The Cradle of Cities.
Nergalner-galThe Burning One — god of war, plague, and the underworldThe Cradle of CitiesErraer-raThe Bringer of Plague — God of Indiscriminate ViolenceThe Cradle of Cities
Why the sources call them one god
Nergal (Sumerian Erra in some traditions, though Erra is sometimes treated as a distinct figure)