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Alastair was a demon appearing on the TV series Supernatural. He served as a secondary antagonist of the show's fourth season. He was played by Mark Ralston in his first two appearances, and later by Christopher Heyerdahl in his last three.
Alastair was once Hell's grand torturer. He tortured Dean Winchester every day for thirty years (in Hell time; the Earth equivalent was three months) and repeatedly offered to spare Dean from torture, if Dean agreed to torture souls himself. For thirty years, Dean rejected Alastair's offer but eventually broke and took to torturing other souls in Hell, unwittingly breaking the first of the 66 seals keeping Lucifer from escaping.In "I Know What You Did Last Summer," Alastair was sent by Lilith to capture Anna Milton, a fallen angel reborn as a human with no memories of her life as an angel. He proved to be incredibly strong, and was immune to Sam Winchester's psychic powers as well as Ruby's demon-killing knife. With great effort, the Winchesters and Ruby managed to escape from Alastair.In the following episode "Heaven and Hell," Anna learned of her true nature and, along with the Winchesters and Ruby, went to reclaim her grace (the part that made her an angel), but Alastair, Castiel, and Uriel were in hot pursuit. It then turned out that Uriel already had Anna's grace. Alastair then kidnapped Ruby and tortured her until she revealed where the Winchesters and Anna were hiding. Alastair, with Ruby and a couple of demon lackeys in tow, arrived in time to meet the angels, who had also found Anna, and a skirmish broke out between the two sides. Although his henchmen were quickly killed by the angels, Alastair proved tougher and was able to beat Castiel to the ground. He then tried to send Castiel back to Heaven, but was stopped when Anna recovered her grace from Uriel and created a blast of energy that destroyed Alastair's host.Alastair resurfaced in the episode "Death Takes a Holiday," trying to break another seal by kidnapping Reapers and killing them on the last night of the harvest. Sam, having grown stronger in his powers, was able to fight back against Alastair, but he fleed. Later, Alastair caught the Winchesters in the astral plane and mocked them about how they normally did the same thing to ghosts (which they had basically become, only they were still alive); however, they managed to break Tessa, one of the Reapers that Alastair had captured, free with telekinesis, and she in turn freed them. Alastair then confronted Dean, but was ambushed by the angels and imprisoned.In "On the Head of a Pin," Castiel and Uriel recruited Dean to torture Alastair to get information as to who was killing angels. However, Alastair enjoyed the suffering. He eventually broke free (secretly helped by Uriel) and proceeded to beat Dean to within an inch of his life, then savagely beat Castiel and prepared to send him back to Heaven but was stopped by Sam, who used his increased powers to kill the demon.In "When the Levee Breaks," Sam, while detoxing from demon blood, hallucinated Alastair torturing him in Bobby Singer's panic room.