Using the Bloodstone Choker, summon and subdue a voidwalker, then return the Bloodstone Choker to Gakin the Darkbinder in Stormwind.
By retrieving my choker and besting Surena, you've proven that you're at least as capable as she was.
With that said, I can tell you're direly in need of training. You can manage the flows of magic, yes, but being a warlock is much more than that.
I will show you how to command a voidwalker, but before I can, you must call one forth at a summoning circle and subdue it using whatever weapons--physical and magical--you have at your disposal. Use the magic of this choker at the summoning circle below.