verb forms - How does the grammar work in "here be dragons"?

Why is the phrase here be dragons not here are dragons? Is this a special grammatical form? If so, what is it?

I found a related ELU question on the topic.

Answer

I thought I'd add this as well, just for those who are inferring that "Here Be Dragons" is an inference on the illiteracy of the scholars during the middle ages. I've separated it out from the other answer because it's not a direct answer to the question.

Anyway, "Here Be Dragons" is actually just an example of Old English (it is invalid modern English)- in particular it's just an inversion of a sentence in the Old English sentence ordering.

In particular

Here Be Dragons

Is an inversion of

[if] here, there Dragons are

In much the same way that in Jack and the Beanstalk the sentence

Be he alive or be he dead, I'll grind his bones to make my bread

is an inversion of the sentence

[if he is] alive or dead, I'll grind his bones to make my bread

Now "If here, there dragons are" is a word ordering that is no longer valid (although the word order should be familiar to German speakers which has the same sentence structure as Old English), and that is why the sentence "Here Be Dragons" is no longer valid.

Note that the subjunctive form in English is still there, so we can construct our own sentences to show that this is still valid English so long as we shuffle the word order around:

I must insist that you be here by 9 am tomorrow for a debrief.

I agree with the recommendation that komodo dragons be here in the public part of the zoo where visitors can see them.

So anyway, long story short is that "Here Be Dragons" used to be valid English, but no longer is. Its use idiomatically is used to insinuate "oldness" rather than "illiteracy".