You have to be apart of the war hammer religion you get the religion if your backstory is apart of the dwarven weapons god backstory. If you are a barbarian and you rage +2 to the damage stats you are using at the moment but -2 to all the rest of your stats you not are using ( this affect does not apply unless you are a barbarian or multiclassed with a barbarian). You can only use the war hammer if you have the dwarven weapons god backstory and your war hammer gets +3 if you are below half health.