"The sound of combat draws you to the scene of a fight between an old Barbarian and an eight-headed Hydra. Your timely arrival swings the battle in the favor of the man, and he rewards you with the pendant he used to calm his mind for battle."
—Pendant of Dispassion pop-up message
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.
This artifact is very useful against hero classes proficient with Fire Magic, as the Berserk's effectiveness greatly depends on how developed the Fire Magic skill is.
Especially the Demoniacs and the Heretics tend to have this skill, and at expert level pretty soon, so the Pendant might be a vital artifact against them. The only other classes with fair chances of learning Fire Magic are the Witches and the two Conflux hero types, the Planeswalkers and the Elementalists.
The Pendant is nothing spectacular against other classes aside for those listed above, but even resisting a Basic Berserk might prove a combat winner. When facing Rampart, Stronghold and Castle heroes (or Beastmasters), the Pendant becomes a mediocre artifact, as the opposition is very unlikely to have Berserk (either due to the fact that Berserk cannot appear in their town's Mage Guild, or that they cannot learn Fire Magic at all). When facing wandering monsters or other armies with no hero, the Pendant becomes totally useless, as no creature can cast Berserk.
Since Berserk is a Mind spell, Undead, Elementals and Mechanical creatures are immune to it. If your army consists mostly of these types of creatures, the Pendant has limited use.