: What is the correct way to use semicolons vs. commas vs. em dashes? In Sabriel by Garth Nix, near the beginning of Chapter Two, there is the following sentence: Anything powerful enough to

I do not agree that this is parallel grammar.
The second sentence after the paragraph is much different than the first. it IS related content and can be condensed by a semicolon, (related by virtue of still talking about "Anything powerful enough to cross the wall") but all the extra baggage after "to become invisible and" applies ONLY to becoming invisible, NOT to assuming the shape of a soldier, and the dash is part of the SECOND sentence, indicating an aside about "mortar bombs" not working, which only applies to the immunity to mortar bombs (and perhaps hand grenades) apparently conferred by "invisibility".
(Perhaps the "Old Kingdom" wind carries some kind of magic that prevents explosive devices from working, I don't know this setting).
Thus the two sentences joined by the semicolon are:

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier.
Anything powerful enough to cross the Wall usually retained enough magic to become invisible and simply go where it willed, regardless of barbed wire, bullets, hand grenades and mortar bombs – which often didn't work at all, particularly when the wind was blowing from the north, out of the Old Kingdom.

Thus the semi-colon is justified to join these two independent sentences that DO have related content, it condenses them. A comma is not; because the grammar of the second sentence, though it starts the same, is much more complex by adding "and xxx" as a consequence of becoming invisible. The use of the dash is again a shortening to not repeat the objects that don't work in the aside that explains why they don't work. This further complicates the grammar and prevents it from being parallel.
The grammar is correct as written (or as the editor rewrote it).

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First of all, that semi-colon is incorrect. It should be a comma. Here's why.

A semi-colon is used 1) to join two independent clauses (stand-alone sentences) which are related in content, or 2) to separate lists of items which have commas, aka the serial semi-colon.

The sentence here has parallel grammar, not independent clauses.

Parallel Grammar

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier; or to become invisible.

This sentence is using parallel grammar. If you extended it, it would read:

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier.
Anything powerful enough to cross the Wall usually retained enough magic to become invisible.

When you have two sentences with parallel grammar, you can condense them, but you do so with a comma, not a semi-colon, because you are removing critical parts of the sentence when you truncate. The condensed pieces are no longer full sentences.

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier, or to become invisible.

(I would even have removed to in that construction, so that the parallel starts at the verbs assume and become.)

Serial Semi-Colon

You can use a semi-colon to separate list items when an "item" is a list in itself. So:

We were offered ham, eggs, and English muffins; bacon, pancakes, and sausage; and grits, cheese, and fruit; and we drank Pepto-Bismol for a chaser.

Nix (or the editor) might have gotten confused with that list of weaponry and thought that because there was a long list, a semi-colon was necessary to attach it to the first part of the sentence. It is not.

The Long Sentence

So fixing that semi-colon (and adding a serial comma later), we have:

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier, or become invisible and simply go where it willed, regardless of barbed wire, bullets, hand grenades, and mortar bombs — which often didn't work at all, particularly when the wind was blowing from the north, out of the Old Kingdom.

The question in my mind is whether the "didn't work at all" applies only to mortar bombs or also to hand grenades. (I've read the book and I'm pretty sure it also applies to grenades, but you don't know that in Chapter 2.)

Because the descriptive clause is lengthy in itself, lengthy at the end of a long sentence, and ambiguous to boot, I would say that the entire construction is too long and should be cut in two so that it's clearer what the clause is modifying.

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier, or become invisible and simply go where it willed, regardless of barbed wire, bullets, hand grenades, and mortar bombs. Explosive weapons, thrown by hand or machine, often didn't work at all — particularly when the wind was blowing from the north, out of the Old Kingdom.

If Nix/editor is completely in love with the long sentence, then we could declare that the clause only applies to mortar bombs, and make it a parenthetical attached to that compound noun:

Anything powerful enough to cross the Wall usually retained enough magic to assume the shape of a soldier, or become invisible and simply go where it willed, regardless of barbed wire, bullets, hand grenades, and mortar bombs (which often didn't work at all, particularly when the wind was blowing from the north, out of the Old Kingdom).

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