Calling

There was a stir within his blood
And the dreams lay thick upon him.
A call did beat within his heart.
One road was left before him.

— Words to a song, scrawled on spare paper, writer unknown

Just as they keep many secrets from outsiders, including the Joining, Grey Wardens keep certain secrets hidden from many of their own members. It is heavily implied that the Calling - also known as The Long Walk - was a ritual created by the first Grey Wardens to prevent future members from watching themselves ultimately succumb to the darkspawn taint.

History

Contrary to belief of the junior members, surviving the Joining does not confer to Grey Wardens a true immunity from "blight sickness," but rather merely delays its onset. This misunderstanding is a calculated lie the senior members allow the new Wardens to believe as one of the oldest traditions of the Order.
The Calling begins with nightmares and bad omens, and then the Warden begins to hear eerie music and whispers, the same "call" the darkspawn hear to search for the Old Gods. Although some junior Wardens claim that it takes a person about thirty years after their Joining to hear it, the gap varies depending on the willpower of an individual Warden and the level of their interaction with the darkspawn. As such, Grey Wardens during a Blight are likely to have shorter lifespans. Otherwise, it is commonly between ten to thirty years that the Wardens hear the Calling.
Several Wardens underwent a premature Calling early in the Dragon Age. Fiona was one of the affected, and was cured of the accelerated taint shortly after the mission into the Deep Roads was completed. The Warden mages at Weisshaupt Fortress speculated that she might permanently be free of the Calling, and considering that she was allowed to leave the Wardens and become Grand Enchanter of the Circle of Magi twenty-seven years after that, it can be assumed that her cure was permanent.

Execution

Some corrupted entities such as The Elder One can use Blight magic to imitate the Call of an Old God, similar to what Grey Wardens hear during their Calling. This can even allow Wardens to be controlled against their will.

Observance

According to ritual, a Grey Warden to hears the Calling celebrates as he or she sees fit before descending underground and entering the Deep Roads to slay as many darkspawn as they can before being overwhelmed and slain. Many within the Order believe that this ritual is done for practical reasons, to take down as many enemies as a Grey Warden can before dying, but the truth is darker. The older Wardens know that the taint will not kill them in the end, but it will transform them into ghouls that feel compelled to join the darkspawn horde, and they prefer to die in battle than to bolster the enemy forces.
It is also implied that even if a Grey Warden would run away, given time, all Wardens would find themselves in the Deep Roads, Blight-Lands, or pursued by darkspawn, for they are all connected by the taint that draws them to each other. Thus, one can assume that almost every Grey Warden will die at the hands of darkspawn, or become a ghoul and part of the horde.