This achievement is Warlock specific and will unlock the Stormcaller subclass.
The questline is given by Ikora Rey in the Tower after completing the first Taken King story mission on Mars (actually on Phobos).
You start by completing a short, linear mission on Mars called "A Spark in Shadow." Then, you must complete a second mission on Mars called "Bottle the Arc." This mission takes place on the Blind Watch crucible map. After defending three "meditation" zones, you become a Stormcaller. You must then defeat Vex with the Stormcaller super ability (which recharges almost instantly) to complete the mission. Then return to Ikora Rey in the tower to unlock the achievement.