The Order of Fiery Promise is a cult which believes the world must end in order to create a better society. They have a strange obsession with the Seekers of Truth and, to a lesser extent, the Templar Order and Chantry. They are known to style themselves in raiments and armour belonging to various chantry organizations.
The Promisers believe that the Seekers of Truth stole their mandate and place as the true Seekers, and in turn, prevented the Promisers from ending the world to allow it to be reborn as a paradise. The Seekers have ostensibly wiped out the Promisers several times in the past, yet they continue to reform and re-appear somehow.
Thedas must be cleansed with fire and reborn as a paradise. This they solemnly promised; they devoted themselves utterly to seeing this come about. Whether they ever drew closer to their goal is unknown.
The Inquisition eventually crushed these "Promisers" during a battle only recorded as the "Cleansing of Churneau." The cult reappeared in the early Divine Age, claiming to take up the Inquisiton's mantle after it had transformed into the Seekers of Truth, leading to Chantry hunts that did not see the Promisers eliminated again until the Exalted Age.—From In Pursuit of Knowledge: The Travels of a Chantry Scholar by Brother Genitivi
—From Codex entry: The Order of Fiery Promise
Dragon Age: Inquisition Edit
In 9:41 Dragon, after Lord Seeker Lambert's disappearance, Seeker Lucius Corin took over his position and continued to lead the rebel Seekers and Templars against the rebel mages. Unbeknownst to others, however, Lucius was now in league with Corypheus; he subsequently abandoned the Seekers and took control of the Order of Fiery Promise. To prevent the Seekers from interfering with Corypheus' agenda, Lucius lured them one by one to be ambushed by the Promisers.
Cassandra later enlists the Inquisitor to investigate the disappearance of the Seekers of Truth. Using the Inquisition's resources, they find out the missing Seekers' location is the castle of Caer Oswin, which they discover has been occupied by the Order of Fiery Promise.
Cassandra and the Inquisitor encounter Daniel, another Seeker who reveals that the Seekers are resistant to the effects of red lyrium, and therefore cannot be controlled by Corypheus. Because of this, Lord Seeker Lucius led them into an ambush by the cultists, who forced them to consume red lyrium, which made them fatally ill.Cassandra and the Inquisitor confront Lord Seeker Lucius, where Lucius reveals his reasons for betraying the Seekers. Lucius is killed in the ensuing battle and the Promisers at Caer Oswin are wiped out by the Inquisitor's raiding party.