Why does "should" come before the subject?
I was watching King Arthur when I came across the strange order of words "should they". This is not a question so why does "should" come before "they"?

Arthur: I'm to be their commander?

Pelagius: Yes. But with this title comes a sacred responsibility to protect, to defend, to value their
lives above your own, should they perish in battle to live your life gloriously in honor of
their memory.

Question

Why does "should" come before the subject?
I was watching King Arthur when I came across the strange order of words "should they". This is not a question so why does "should" come before "they"?

Arthur: I'm to be their commander?

Pelagius: Yes. But with this title comes a sacred responsibility to protect, to defend, to value their 
                  lives above your own, should they perish in battle to live your life gloriously in honor of 
                  their memory.

Answer

You'll find this form in business correspondence as well, so it's worth knowing how it works. It's used in this dialogue to give a formal "old English" tone to the speech.

You can understand "Should they perish..." to mean "If they should perish..." or even "If it happens that they perish..."

In this case, "should" is not advice or a suggestion. It's used to present a hypothetical situation.

You'll find out later how reversed word-order is not only limited to questions.