It's a pair of old-fashioned idioms, be it and ever so.
- Be it is used here to mean even though it might be.
- Ever so means extremely.
All together, the phrase means: even though it might be extremely severely.
It's an older style of talking. Few people speak or write that way today, but there's nothing wrong with it. More examples:
- I will win, be it the last thing I do. = I will win, even though it might be the last thing I do.
- They were ever so tired. = They were extremely tired.
Does that make sense?