Something entirely new to a person from battle.net would be monster counters. Just as a player can have chance to cast skills on struck and dieing from gear the monsters can now do the same. Many common monsters have when struck counters and/or death counters. Some counters are mild on damage yet on the other hand some monsters can have very powerful counters such that a player would often flee from it before they get completely trashed. Boss monsters always have some sort of counter. This poses a challenge that often must be met with a strategic analysis of the enemy, often a little understanding will be the difference between an easy win and a frustrating death. A common misconception taken into Aftermath is the idea of attacking with careless abandon, in several instances a controlled and careful approach will yield a much better result. Check the boss detailings and strategies for more information on dealing with counters.