It's a subjunctive use of the verb to go with the speculative "If". English speakers don't use the subjunctive as often as some other languages but it does it exist. E.g.
"He will not win, be he ever so brave". (implying "even if he proves himself to be ever so brave")
I can't think of another negative one off hand, but I'm sure the quoted example has parallels in similar literature with its use in a sort of incantation or proverb. Such usages sound quite formal and the negative formation in particular does sound somewhat antique, which fits the context quoted.
A common phrase that uses the first person subjunctive is:
"I wouldn't do that if I were you."