Well, this is a little terrifying. Very impressive designs as always. Based on the name, am I correct in assuming Neptune's Maw is a water-themed dungeon?

In my opinion, the border between the dark blue areas on the top left and right and the lighter blue areas below them is a little too much of a straight line. Unless there's some sort of animation effect only visible in the game itself, you might want to make those borders a bit more ripple-like or diagonal.
Also, I'm not sure what the green spire in the middle of the screen is. Maybe I'm missing the obvious here, though.

By the way, what's up with poor Reyson? That expression on his face is more than a little concerning. He must have seen some really bad stuff recently.

author=NeverSilent
In my opinion, the border between the dark blue areas on the top left and right and the lighter blue areas below them is a little too much of a straight line. Unless there's some sort of animation effect only visible in the game itself, you might want to make those borders a bit more ripple-like or diagonal.
Also, I'm not sure what the green spire in the middle of the screen is. Maybe I'm missing the obvious here, though.

There is indeed a bit of movement in the background, as the background-image swirls and moves a little. I drew the images that appear between the background swirl and the battlers, to give it a "lightning" theme.

The "spire" in the middle is supposed to be the transparency starting at near-zero and increasing as it expands horizontally, but I've learned more about making these sort of things look better since then. If it still looks wonky in-game, I'll redraw it for the final release.