The Drowned City
After an apocalyptic explosion of magic in a formerly magic-less world several centuries ago, New Orleans has become a forbidden land, guarded by a menacing warden ready to make a meal out of unwary travelers.
Update - 2 May 2020
As I was painting the final image, I started to expand on the world-building.
I decided that a decent chunk of people had survived and had them utilize old structures, a skyscraper, a ship, and a construction crane to build themselves a new "town."
I thought about how the Apocalypse was caused by an abrupt explosion of magic into a formerly magic-less world and how New Orleans was the epicenter of this reality shift—an idea I had already been playing around with once I had decided to paint a giant crocodile and a post-apocalyptic New Orleans.
I later decided that the Ancient Guardian would be artificial and made of metal because it was built by these survivors using both magic and some of the remaining technology they had from before the Apocalypse. Occasionally, curious outsiders would launch expeditions to figure out what caused the Apocalypse—unaware of the Ancient Guardian ready to defend the survivors and the city's bones.
I knew I wanted to explore something different for "Forbidden Lands, Fallen Empires, and Ancient Guardians" and went through a lot of brainstorming. I wrote and discarded notes for different settings and themes—including a more northern and "viking" type of setting, something from Babylon/Mesopotamia, and to a more futuristic era.
Eventually, I started thinking about Ancient Egypt and alligators/crocodiles and the goddess Ammit and how I could spin that so it was something new and interesting.
I decided I didn't want to do a "Fallen Empire" in the desert, which eventually led me to think of swamps. From there, I made the connection to alligators/crocodiles, Ammit, Ancient Guardians, New Orleans, and swamps and started doing thumbnails!