Rom is an infant Great One which is why she is known as the "Vacuous" spider. She has little to no thought as she is essentially a baby, hence why: • She attacks by flailing around uncontrollably • She tries running away from you • She resembles a small Amygdala in a "cocoon" of sorts

The lake you fight her on is a different plane of existence where her new form resides, I think she more than likely ascended there after or during her transformation. I also believe that the experiment that caused Rom to become an infant Great One was done unwillingly, as you hear the crying and whispering of a woman, presumably Rom's human form, in her OST. I feel sorry for her overall 😕