Originally Posted by
Tavrobel
Auras from skills and aura from weapons stack. They both activate at roughly the same time, but due to the nature of DII and internet play, it can take anywhere up to five second for either one to activate. Usually, Auras switch animation (unless by the time the other one will show animation, it will have ended, in which case, it doesn't show at all).
For example, a Barbarian casts BC and BO on a Paladin with Concentration and Azurewrath Blade equipped. The animation will switch from one to the other based on priority. It will at first show Conc, then Sanc, and then BC or BO based on order of casting. Usually.
Baal, Diablo, and Mephisto are not immune to anything. They just have hella lot of HP.