What is even happening...

Right now, Code Hell's body is similar to that of a half snake person. Instead of a snake's tail however, her tail was made out of a solid black gas-like substance that moved freely like clouds in the wind.

The cloud of shadow, indeed, it had become solid. This happened, after Code Hell had returned to her physical appearances again but only on her upper body. It had taken a few short seconds before the shaped cloud of shadow became solid though.

This, I think it was all according to the plan of Code Hell. I'm saying this, because Code Hell had only made the shaped cloud of shadow solid after having the shadow wrapped around the body of Stacey like a python's tail would do.

It was exactly after the shadow had already surrounded the body of Stacey, when that shadow became solid, solid enough to look like the tail of a purely black snake.