All this data is potentially out of date, and should be taken with a truckload of salt
The Udorians are one of the many races conquered by the Amarr Empire. They originated on 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.