Herald of Undoing

Other creatures can't attack.
At the beginning of each player's upkeep, that
player may pay {3} or exile another creature they control. If they do, switch Herald of Undoing
power and toughness. It loses all abilities except
this one, then it gains ''Other creatures can't
block.'' If Herald of Undoing had that ability at
the beginning of the current turn, instead it
gains ''Other creatures can't attack.''