Monsters in Judgement are a neutral antagonist to both players. They can really hurt and hamper your plans, but the best players can use Monsters to their own benefit via board placement and control, and the ability to move models around via Combat Manoeuvres or certain Active Abilities.
Monsters have their own activation in the Communion Phase which happens at the start of each turn. In this activation they will advance their Mov (usually D3) in inches towards the nearest hero. If they finish their movement and find the nearest Hero in range of one of their attacks, they will attack! This is the only time a Monster will activate, but not the only time that they will attack…
They also attack any time a model advances and ends their activation in their melee range as well as any time a model is pushed or placed into their melee range, or consequently, any time the Monster is pushed or placed into melee with something else! Short version; Monsters hurt and are dangerous!!
But they aren’t all bad. When a Hero kills a Monster, that Hero gains a level as well as the Fate Bounty printed on the Monster’s card (the Fate is only collected if the attacking Hero is within 2″ of the Monster). The level and Fate can easily be deciding factors in a match, and so this adds another level of depth to the game; either with racing for the first Monster kill to try and get ahead, or with looking for opportunities to “kill steal” a Monster that your opponent was banking on killing themselves.
Monsters do respawn on the turn after the turn they are killed, so you do get some breathing room after killing a Monster – but don’t forget about it because a Monster respawning in your back lines if you aren’t prepared can wreak havoc!