The invisible circle

Seven people receive invitations to a singular experience during a weekend in Cornwall in a castle reputedly built on the site of King Arthur's, and situated on an island tenuously connected to the mainland. The sinister host announces that a murder will be committed that night, shows them a sword embedded in a stone --which none of them is able to pull out --and a golden chalice which he claims is the Holy Grail sought by the Knights of the Round Table. Thus begins a nightmarish series of events including impossible murders, the disappearance of the Grail, and communication with the mainland cut off. Has the vengeful king indeed returned? And is there any connection to the recent release of the host's homicidal half brother?