The Grammaring Guide to English Grammar with Exercises

Subject-auxiliary inversion after SO + adverb / adjective and SUCH

When adverbial or adjectival phrases starting with so are placed at the beginning of the sentence for emphatic effect, the subject and auxiliary are inverted:

So quickly

did he run

that the others couldn't catch up with him.

(He ran so quickly that the others couldn't catch up with him.)

So strong

was the wind

that we couldn't open the window.

(The wind was so strong that we couldn't open the window.)

So + adjective can be replaced with such:

Such

was the wind

that we couldn't open the window.

(The wind was such that we couldn't open the window.)