The last boss, when a mob spawns, does a sunder effect on the tank, eventually leading to shattered armour. I can see from the logs that I am taking more and more damage.
In trying to gauge the armour loss and how many hits of sunder I can take I looked at my stats screen while having this effect but it does not show a reduction in armour like when other mobs that do sunder - the mobs in MoL for example.