You will need:
The NPC that gives this quest is located in heroic Slave Pens. Just after the first boss, after dropping down into the pool, he is in a cage on the left side of the next room. The NPC will not give the quest to players above level 70 anymore!
Bring me the elemental signets that the cudgel requires!
The earthen signet is held by Gruul the Dragonkiller. Gruul can be found inside his lair in the Blade's Edge Mountains.
The second signet is the blazing heart of a creature known as Nightbane. This mythical beast is said to reside in the hallowed halls of the prophet, Medivh.
Upon returning to Skar'this the Heretic, he will cast a spell on you allowing you to enter Serpentshrine Cavern.
With the cudgel reformed, I shall break free of my bonds and return to Serpentshrine to exact vengeance on Vashj and her minions.
I have not forgotten our agreement, mortal. You too will now be able to enter the lair of Lady Vashj. Now leave me! I have much to plan.