Sarras was ruled in the Grail histories by King Evalach (Evelake), whom Joseph of Arimathea converted to Christianity. According to the Vulgate Queste del Saint Graal, the Saracens took their name from the city. (In truth, the name of the city was likely invented by the author to explain the word Saracen.)
Galahad, Perceval, and Bors journeyed to Sarras with the Grail at the end of the Grail Quest, and found that it had reverted to paganism. Its king, Escorant (Estorause), threw the Grail knights in prison for a year, but released them on his death bed. Galahad, against his will, was chosen king, but he died himself within a year. Its main temple, the Spiritual Palace, served as Galahad’s and Perceval’s burial place. The island as a whole, however, does not seem to have been fully converted until Galahad’s reign, if then.
The Quest of Galahad and his companions ended in Sarras, and from Sarras the Grail was taken into Heaven permanently after Galahad’s death. Sarras had religious and mystical associations at least from the time of Joseph of Arimateha. The Isle of Man and the Lake District work very well together as the Sarras and Waste Lands of the Grail Adventures.
The fact that the knights are thrown into prison suggests that Sarras was a political entity as well as a mystical place, and that the secular government was unfriendly to Arthur. If Sarras is identified with Soleise, then Estorause must have succeeded the king whom Arthur and his men killed on the banks of the Humber.