What did rose see in the cueball(or what its called, the thing where you can see answers and what Doc Scratch was prototyped from) that made her grimdark? And didnt Vriskas ancestor have it first?

There are multiple cue balls that have been dispersed by Doc Scratch. She asked the cue ball whether the Horrorterrors were benevolent and this caused her to go grimdark. What she saw wasn’t shown but the answer was obvious: they’re not.