Whoever — anyone at all who does something, with no exceptions. 'Whosoever believes shall be saved.' 'Whosoever enters must sign the book.' The meaning is exactly the same as 'whoever'; the difference is entirely one of feeling. Whosoever is old and grand. It is the language of the King James Bible, of wedding services, of old laws and contracts and wills. Adding that '-soever' does one job: it slams the door on exceptions. Not just 'whoever', but 'whoever it may possibly be, no matter who'. That is why lawyers liked it, and why it can sound solemn or thundering. Today you will meet it in three places: old religious and legal texts; fixed phrases like 'to whomsoever it may concern'; and modern writing that is being deliberately grand or gently joking — 'whosoever ate my lunch, please confess'. It has brothers built the same way: whatsoever, whensoever, wheresoever, howsoever. Of these, only 'whatsoever' is really alive in normal English, and mostly after a negative: 'no reason whatsoever', 'nothing whatsoever to do with me'. For your own speaking and writing, use 'whoever'. Keep whosoever for reading.