The Inquisition was reformed in response to the violent and unprecedented murder of those that attended a Conclave to broker peace between the Templars and the mages. Its intention was to determine and destroy the guilty party who had killed so many, and bring peace to Thedas.
A Darkspawn named Corypheus intended to destroy our world as we know it, creating tears within the universe to allow demons to pour through from the Fade. With Asala Adaar as our Inquisitor, we were able to stop him.