I do not know what "to fail at one's peril" means. The phrase appears in legal documents until the 19th century (at least this is what Google suggests). I cannot deduce its meaning no matter how hard I try.
closed as general reference by FumbleFingers, Gnawme, kiamlaluno, Matt Эллен, Mitch Feb 10 '12 at 2:53
This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information. See the FAQ for guidance on how to improve it.
The complete sentence is often something like, "Whereof you may not fail, at your peril." This means that if you fail, it will be perilous for you.