He was a big one, to be sure. At his belt he had three calves strung up by the heels, and he unhooked them and threw them down on the table and said: 'Here, wife, broil me a couple of these for breakfast. Ah! what's this I smell?
I smell the blood of an Englishman,
Be he alive, or be he dead,
I'll have his bones to grind my bread.'
This content is from "Jack and the Beanstalk"
Why is "be" used instead of "is" in this sentence "Be he alive, or be he dead"?