Hmm, I can tell you spent a lot of your time brainstorming. Unfortunately, there isn't much of what you included in the GDD yet, so I can't review too much.
For a first prototype, not bad. I recommend explaining why we start with voidrunners- that is completely unexplained in the game, as you only introduce our main character. Also, there is no maze, just a slime bossfight- which need to be balanced to be a LOT weaker, especially for the first boss.
I'm not sure if this is intentional, but when I finally beat the boss, it just led me to the loading scene picture. I cannot control anything further.
When/If you make the main character sprite, make sure they are outlined in a colour that floor tiles do not use/ do not use often. With a limited pallete like this, that is difficult, but important to see our character wherever they go.
Spelling errors were common- AbysSlayer, for example. When naming these runners, seperate their names like "Abyss Slasher"
When there is only one target, like the slime, the attack button should select that target automatically. That will make the game a lot quicker and reduce frustration.
Good music choices! Very Abyss-y.