In the System Prompt section. It's not the description box, but the box lower down on the first tab when you edit a world.
You should *also* either define your terms there, or use the Dictionary tab. You are using terms that I am not familiar with (and I don't really need a definition). You should define these in some way that the instructions to the AI knows what your intent is.
Short definitional phrases are _usually_ enough to inform the AI, but when in doubt, also provide examples in your prompt so the LLM can decode how you want it to conduct your game. I had to do this with a corruption system because the AI was adding ludicrous amounts of corruption every turn, instead of only in the specific scenarios where it was supposed to be added.
For the "Digestion" mechanic, you can *probably* just keep the instruction simple. Something along the lines of "Threshold: if > 100%, the player starts losing health" might work.
And yeah, if you don't want certain creatures to gain certain traits, you should be able to just clearly state that in their description on the Entities tab. "This creature can never have wings" or something to that effect.