syntax - Reason for Subject-Verb Inversion: Only in cases where A is B, shall the Company do X

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.

“In no event” and “under no circumstances” seem to be prepositional phrases, yet 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?

Answer

It's grammatical.

Subject-verb inversion is required when preposing a negative adverbial of time, place, or circumstance.

• At no time did he say that. ~ *At no time he said that.


• Under no circumstances may she enter. ~ *Under no circumstances she may enter.

It is not allowed, however, when preposing other adverbials.

• *With no hesitation did he speak up.


• *With no grace did he accept it.

Only is a negative.