The Dragonriders were King Vendrick’s royal guard. Together with the king, they helped found Drangleic by crushing its former inhabitants and erecting the kingdom upon their graves. Long ago, the Dragonriders mounted wyrms and were feared on the battlefield for their unparalleled strength. The armor and weaponry they utilised demanded great skill and inhuman strength to wield. Those who aspired to join the Dragonriders were trained with their wyrms. Those who did not have the mettle to handle their training were torn apart by their wyrms, while those who survived emerged with deific strength. The weapons of the Dragonriders were imbued with magic and included a halberd, a twinblade and a bow. The halberd and bow are likely to have been used atop their wyrms as they enable long reach.
The Dragonriders we encounter in Drangleic are found at the Tower of Flame in Heide and the royal treasury of Drangleic Castle. The Dragonriders were clearly trusted by Vendrick and seem to be placed either to protect the areas they are in, or to oppose any undead who would come after the King. The Dragonriders are weak to fire and lightning damage but stronger against magic and dark, indicating that they have since hollowed.
What happened to their wyrms is unknown, but it may be that the creatures have since died out, while the Dragonriders endured because they have hollowed.