Imagine that Shudderwock battlecries trigger twice. Imagine that all of your battlecries trigger twice. Sounds good, right? Well, that's the reward you get for completing a new Shaman quest.
Of course, you must pay two mana each turn to activate this ability, since it works like a hero power. However, if you have the coin, or you managed to reduce the cost of Shudderwock with the help of Far Sight, then it just becomes twice as powerful.