More informative catch-up posts about the Dragon Age series, in case anyone wants to play Inquisition without wading through the first and second games first.
The Dragon Age series has two major threats that your protagonist must face:
The first threat is the Blight.
This is a historical period in Thedas where darkspawn demons takeover the Old Gods, resulting in an Archdemon. This Archdemon leads hordes of demons to cross from their world (The Fade) to the tangible world.
The first game takes place during the time of the world's Fifth Blight, and your character (a member of the once-prestigious order of the Grey Wardens) must defeat in the Archdemon and save the world.
In the background of the first game, we are first introduced to our more apparent threat:
The Mage-Templar war.
In Thedas, the Templars are an order of knights tied to the church, the Chantry, which are used to control and look after the mages. The mages are confined, by law, to The Circle-- a college where mages are trained but also caged.
Mages who escape or evade the Circle are called Apostates, and are hunted down and murdered by Templars.
The war is meant to determine the rights of the mages-- whether they will remain under the eye of the Church, or whether the Circle will become independent from the Templars and the mages will be free to roam Thedas as they choose.
This war has caught a lot of people in the cross-fire, destroying villages, creating refugees, and killing countless innocent people.
This is where the second game focuses its gaze; the Inquisition is a new order, meant to mediate between the mages and the Templars both, to diffuse tension, and help those caught in between.