Rider of Black is the Rider-class Servant of Selenik Icecol Yggdmillennia as part of the Black Faction in the Turifas Holy Grail War of Fate/Apocrypha.
Black Rider's identity is Astolfo, the Twelfth Paladin of Charlemagne.
In the Charlemagne Legend, he is the son of an English king, and one of Charlemagne's twelve faithful Paladins. He is beautiful beyond all compare, but he is also a frightfully unparalleled blabbermouth, prone to speaking on and on until those involved lose all rationality. His conversations, through no ill intent on his part, have a way of meandering toward maximum social awkwardness. His hobby of adventuring gets himself mixed up in an assorted amount of mischief due to him constantly popping up everywhere. He states that his hair ornaments, which seem like something a princess would wear, are an "irresistible proof of friendship" that he uses to restore peace to his depressively mad boon ally, Orlando.
(Source: TYPE-MOON Wiki)