You gain the ability to wield the magic from the Magicka games. With this power, you become a master of elemental forces, able to use and combine the elements of Water, Lightning, Life, Arcane, Shield, Earth, Cold, Fire, and the sub elements of Steam (Fire & Water), Ice (Water & Cold), and Poison (Water & Arcane)
You gain the ability to wield Magicks, powerful spells with more abstract effects than raw elemental destruction, such as Reviving people from the dead (helpful when you blow somebody up by accident), Nullifying changes to the environment (helpful for cleaning up all the property damage you're going to cause), randomly instakilling someone in the general vicinity (possibly including you), or Summoning a Napalm Airstrike from thin air (no, really.)
You can also create any of the staffs, robes, or weapons found in the games or DLCs