Sir Elyan the White or Helyan le Blanc is the son of Bohort by an unnamed daughter of King Brangor. The Prose Lancelot and some later texts say that Elyan afterwards became Emperor of Constantinople, but this story of Elyan has not survived.
Elyan is brought to court by his father Bohort when Elyan is still a young lad and is made a Knight of the Round Table shortly before the beginning of the Quest of the Holy Grail.
Galahad, son of Lancelot is said in the Quest of the Holy Grail to be the last surviving member of King Solomon’s lineage to which Galahad belongs through his descent from Lancelot's mother, Elaine. Elyan must be of similar descent, as Evaine, his father Bohort’s mother, was Elaine’s sister. So logically Elyan must be imagined to have had a short reign as emperor, and to have died before Galahad.
According to the Vulgate Merlin, King Brangor’s wife was an unnamed daughter of the Emperor Adrian of Constantinople, which may be a detail from the lost story of Elyan, or may be invention to explain that Elyan, as grandson of the Emperor Adrian, had a right to inherit the imperial throne.
The Post-Vulgate Quest of the Holy Grail lists Elyan the White as one of the twelve Knights of the Round Table to share in the final Grail banquet at Carbonek.