Haven't tested the boss yet but seen a lot of videos, and it looks just like altpowerbar to me.
The video i checked was from one pov so same UI, and when they were doing stone guards his altpowerbar was showing in the same spot as this one.
Not much of help, what im trying to say is that i think it's the altpowerbar controlling it, though like p3lim said you also get pet/vehicle actionbar showing so i might be wrong.
