The Pendant of Dispassion grants immunity to the Berserk spell to all the creatures in the hero's army. Enemy heroes can still cast Berserk on the immune creatures, but the spell will have no effect. What increases the artifact's strength is the fact that the opponent's hero won't be warned (as they usually are by a message, when there is no valid target for the spell they are attempting to use), so he/she might waste a turn and some mana casting an useless spell. This alone might give a tremendous advantage to the Pendant's owner.