The Dargon King in his second and third forms can cast Judgement, which appears to reduce all your heroes' hit points to <5%, debuffs all your characters except for Regeneration, and has a chance to nerf your characters.
Fortunately, after using this ability, the Dargon King is reduced to the bottom of the action meter (i.e., this takes a long time to recover).
This ability goes through armor (i.e., Full Armor Tier IV Level 11, etc. will not save you). However, a Guard will reduce the damage taken from Judgement, though you will still want to heal afterwards, making this a nearly moot point.