The Udorian are one of the many races conquered by the Amarr Empire. They originated from Amarr Prime, along with the True Amarr and Khanid. Their arrival on Amarr Island is what spurred the launch of the Reclaiming.
Though originally distinct from the True Amarr, they are now almost physically and socially identical, and impossible to tell apart. Their status is considered nearly equal to the True Amarr by all but the most conservative Amarr elements. One of the royal houses, the Tash-Murkon Family, is of Udorian heritage. The current empress, Catiz I, is also a Tash-Murkon.
Even though most Amarr accept the Tash-Murkons as equals, there's a minority that believes that only nobles of True Amarr blood can become the Emperor. A short time before Catiz I's coronation in YC 118, a group of loyalists refused to accept her claim to the throne, and formed an army known as the "Purity of the Throne", with the objective to stop the coronation.