Situation: Wizard has Suppression Cloak and has Guard marker.
Enemy creature ( e.g. Timber Wolf) that is in the same zone as Wizard is going to make an attack, but Beastmaster doesn't have any mana.
What now ?
a) Timber Wolf may attack another creature in the zone;
b) Timber Wolf cannot attack any creature in that zone; ( TW must attack Wizard, but there is no mana in Beastmaster's mana pool).