The Stormcaller is one of the new subclasses included in Destiny: The Taken King. Getting a new subclass is not so immediate as one could expect, so read our guide in case you are encountering troubles in unlocking it. First of all, you need to beat the first part of The Taken King main quest. Return to the Tower and get the quest "The Stormcaller's Path" from the Warlock Vanguard Quartermaster. You will have to play in solo mode, and face Vex and Cabal.
Use your Ghost to discover where to go and reach the barrier. There you will find three elite Hobgoblins spawning, kill them and you will open the barrier. Once you go through it, you will find a large Vex enemy with a shield: beating him will allow you to access the second part of the quest, "Bottle The Arc".
Go to three points and meditate – don't worry, this won't be boring as you will have to defend each time from a new wave of enemies. This will unlock the Stormcaller subclass, which you will have to use instantly to get save and sound back at the Tower, where you will get an extra reward.