[slomikmw] Promotion to king

Consider a chess variant where a pawn can promote to king.  The opponent must capture all kings (checkmating the last one) to win the game.

Is promotion to king too powerful a defensive technique?  If so, consider some fairy chess inspired counterbalances.  A pawn may also promote to amazon.  The pawn does not promote to king per se but to a royal ferz, a colorbound piece with half the available moves as a king.