Corypheus was once one of the Magisters Sidereal. In Dragon Age: Inquisition, it is revealed that the Grey Wardens are particularly vulnerable to corruption by Corypheus. While this was originally a bitter-sweet clause in the Grey Warden contract, Dragon Age: Inquisition revealed just how vulnerable to manipulation the Taint made members of the Grey Wardens. It eventually leads Grey Wardens who live that long to go mad and wander into the Deep Roads, overwhelmed by the same compulsion to seek out and corrupt the Old Gods that drives the Darkspawn. The Calling takes place between 10 and 30 years after the Joining ritual according to Alistair in Origins. However, it also eventually leads to the Calling. This gives them the ability to sense Darkspawn and receive the psychic transmissions of an Archdemon. However, in order to become a Grey Warden initiates must drink Darkspawn blood and allow themselves to become corrupted by 'the Taint' that affects all Darkspawn.
Only a Grey Warden can slay an Archdemon, ending a Darkspawn Blight. The Grey Wardens have always performed a delicate balancing act.
