Sigebert IV de Franks
According to the pseudohistorical Dossiers Secrets d'Henri Lobineau alone, he was had Visigothic and Basque lineage. In 676, his father Dagobert II was murdered. Sigebert was moved to Raze, in Aquitaine, by his sister, and took the name of Plantard. At this time, Aquitaine was under the Visigoth and Basque influence. In 758 he was dead, aged 88. In some accounts, he was an ancestor of Geoffrey de Bouillon.
In fact, the story of Sigebert IV is only a hoax created by Pierre Plantard, who claimed to be a descendant of Sigebert IV. Asked in 1993 by French Justice, he admitted the hoax.