Arbiters are mages who have studied spells of invulnerability to perfect the art of magical damage mitigation. Armed with the powers of Water and Air, they taunt and torment enemies with storms and brush off their attacks with shields of ice. Don’t be fooled by the cloth and wand – these mages stand toe-to-toe with Riftstalkers, Paladins, and Justicars on the front lines of battle.
More about the Lore of the Arbiter is available HERE.
Apparently the mage tanking powers were granted by a mysterious giant Corgi, imagine that!
Also, if you’re looking to experiment with the new souls in builds, Magelo has put up a soul calculator (abilities subject to change, of course) http://rift.magelo.com/en/soultree/pts