<Li> Bleu Landau as Blue, Back Lack's son . </Li> <Li> Mikael Persbrandt as Greybeard, a viking who has deals with Vortigern . </Li> <Li> Poppy Delevingne as Igraine, Arthur's mother and Uther's wife: she is killed by Vortigern in the beginning of the film . </Li> <P> Kamil Lemieszewski, Jacqui Ainsley and Rob Knighton portrays Merlin, the Lady of the Lake and Mordred, three important characters from the Arthurian Mythology, respectively . Katie McGrath and Millie Brady portrays Vortigern's wife, Elsa, and daughter, Catia, respectively . David Beckham appears in a cameo . Lorraine Bruce, Eline Powell and Hermione Corfield appears as the three Syrens . Michael McElhatton portrays Jack's Eye, Arthur's scout on the kingdom . Nicola Wren portrays Lucy, the prostitute who raises Arthur . </P>

King arthur legend of the sword arthur's mother