The Crusaders or the Knights of Stendarr is a knightly order that works under the Temple of Stendarr, a group dedicated to Stendarr of the Nine Divines. They serve as the enforcers of Stendarr's will, they can be seen throughout sects of the Temple and the regions of Tamriel.
The Crusaders were active during the Interregnum, protecting the innocent and enforcing the will of Stendarr. They would send members to areas throughout Tamriel, to quell danger for the good of the people. In 2E 582, the Crusaders had sent Crusader Dalamar to the region of Craglorn in Hammerfell. The ruined city of Skyreach had been the source of legends and myths throughout Craglorn, most of which have been revealed to be true. Dalamar was sent to the Skyreach Catacombs, when news came of dark magic had spewed from the area, the Crusaders took action. Dalamar was unable to control the tomb, prompting him to call upon the help of adventurers in the area. Eventually, the Skyreach Catacombs was secured, but Crusader Dalamar remained in the tomb, to ensure no further disturbances.
The Crusaders were active throughout the third era, having a presence in the Iliac Bay. They can be seen throughout the Alik'r Desert and the Barony of Phrygias, typically in the jurisdiction of the Temple of Stendarr, doing what they do best. Not much can be said about the Crusaders at this point. It was likely present during the Oblivion Crisis, fighting against the Daedric Invasion. After the event, another group had arrived called the Vigilants of Stendarr, which served as a militant order that fought against Daedra. This group had dispersed sometime in 4E 201. It is unknown whether the Vigilants and the Crusaders were related, though there is a possibility. It is most likely that the Crusaders had remained active along with the worship of Stendarr.