Behest is an authoritative command or request. If your boss or principal asks to see you, you go to their office at their behest.
Behest rhymes with "request" and they almost mean the same thing, if you give your request a little authoritative oomph, just a tad bit of "or else." Another difference between these words is that you can't use behest as a verb: You can't behest someone. You act according to their behest. The root word here is the Old English hehaes, means "a vow."