subclass and dualclass question + chaos essence

1. I have just awakened to Tyrr Doombringer and I want to create a subclass. But i dont know whether different subclasses give different skills to main class. If they give different skills, what sublcass can you advice. If all subs give same skills, can you advice which nuker/bow/dagger classes would make great dual class, or should I make dual an iss/summoner.



2. 30 day chaos essence pack allows me to change my class for 30 days or change is unreversable?