She'd known this day would come, known that the cult hunting her would track her down like they'd tracked down her mother, would try to open the lock to which she is now the key -
She'd hoped her child would be grown, first.
The shoggoth picks her child up, cradling the seven-year-old to her chest and whispering, "Hide your eyes, Bright," and turns one of her human arms into a long, serrated tentacle.
The cult is smarter than to rely solely on physical force, though, and their magics close on her, binding her -
Something slithers through her mind, and the world inverts. When it settles, she's elsewhere, still clutching her shaking child.
Instinctively, she reverts her form to an entirely human one.