I'm no big brain dev, but thought I'd leave a bit of feedback. Hopefully it'll be of some use to you!
Like many others have said in the comments below, the artstyle for the characters is really interesting and the music choices are fitting! The font fits as well, though it can be a bit hard to read from time to time.
I do appreciate how you can basically use mouse only if you wish to (not saying you need to stick with that though, just thought it was interesting!).
A suggestion would be to have an indicator of how large Muriceps' attack is! I was pleasantly surprised with how generous the hitbox was as I thought the attack range would be quite small, considering it's like a punch. Maybe something similar for the rats too? Another thing to point out is that the punch noise might be a liiiiittle too loud.
Another suggestion would be perhaps for a run button! Chasing rats can take quite some time, and especially in that last really big dungeon, when you have 1 to 2 rats left, it can take quite a bit to traverse through the rooms.
Whilst I understand you'll tune the references down if you continue to work in this game, in an overall the humor was nice!
I also played the nefarious mode, and the rats chase you when you get too close, which was a nice surprise! One thing I'll point is that the distance they can damage you from seems to be quite far. There were times I'd kill a rat and receive damage. I do appreciate that you gave +2 lives after the first wave of rats in the last dungeon room, though. Also, extra modes with differences between them? That's hard stuff to do! Good job!
This is really good work, especially considering it was your first experience with Godot! Can't imagine how scary it'd feel to go into a gamejam using an engine I don't have any experience with.
Also, yes. I'm a fan of the battlecry button. I like it. Quite a bit.
Well done! It'll be interesting to see what you do with the experience that you've gained making this game! Good work!