Tempests are versatile, able to attack from long range or channel lightning into a short-range destructive burst. They also possess illusory tricks to help evade defeat in the event that heavy armor isn't protection enough. Despite this, they're probably the least adept of all the Warrior souls at wielding non-lightning weapons, which means that another soul is necessary to provide hard-hitting weapon talents. Take a look at the introductory lore and early screenshots on the official reveal page, and keep your eyes peeled for the final two souls as the expansion nears release.
[Thanks to Brad for the tip!]