On Heroic difficulty, when any of the three adds spawns during the fight, they will immediately cast Thunderous Crash, dealing damage to all players and knocking them back. This effect occurs no matter where the player is standing in relation to the summoned add, and is most dangerous during the Oppression Phase since players will have to move in order to reset their position.
Additionally, the Ancient Harbinger in Heroic difficulty is buffed with Impatient Mind. Each time players interrupt Harbinger's Mending, the Harbinger gains Impatience, which causes the next cast of the spell to be cast 15% faster. This effect stacks, so eventually the Harbinger's Mending cast time will become too quick to reliably interrupt.
These new mechanics do not significantly change the mechanics of the fight, but players will have to be more coordinated and careful in interrupting the Ancient Harbinger.