Nowhere, Never
Nopony.

...

Wait why is somepony here?
It shouldn't have started, i wasn't ready

Twilight started to converse with someone whom she shouldn't be able to hear, uncovering truths she sould have never learned. And Celestia is definitely behind this.

How to react?

Isekai'd? not really.
A way to use my scraped idea for an AU? totally!

Twilight just came back from canterlot where she was helping to reorganise Equestria after the Storm King attacked. She was given a strange and old artifact seemingly dangerous, and as she try to purify it, unseen circumstances will uncover hiden and forgotten parts of history, and enemy will rise to take profit of the chaos that unfold

Starswirl will have some explaining to do.