The objective ( formerly dative) case of who.
Whomsoever.
At any rate, after that there were friends to whom one might go- after this?... – Fortitude by Hugh Walpole
Then there were others whom she did not know at all. – Betty Lee, Freshman by Harriet Pyne Grove
But you must take after your mother, whom I never saw. – In Luck at Last by Walter Besant