In the following, why does subject-verb inversion occur? Is it necessary? And what is this type of inversion called?

Colleague’s original: Only in cases where A is B, the Company shall do X.

I changed to the following: Only in cases where A is B shall the Company do X.

Searching Google for “shall the Company” gives examples such as:

In no event shall the Company . . . Under no circumstances shall the Company . . .

And these all seem quite natural.

However, “in no event” and “under no circumstances” seem to be prepositional phrases, but I would say simply, with no inversion:

In the fridge, you will find some beer.

Is the S-V inversion maybe some sort of archaic style that remains in legal or maybe religious texts? Perhaps a remaining German-style syntax?