Picasso with a razor is how Alastair was aptly described by Ruby in the fourth season. He only appeared in five episodes, and portrayed by two different actors (Mark Rolston and Christopher Heyerdahl) but yet he is still remember as one of the best villains on the show. He was a white-eyed demon who was the grand torturer of souls in Hell. He also is one of the more powerful demons met in Supernatural, as he can send an angel back to Heaven, also he is immune to Sam's powers and Ruby's demon killing knife. Surprisingly, he showed in the episode "Death Takes a Holiday", that he had knowledge of the Four Horsemen of the Apocalypse. In that episode, he was breaking a seal by killing reapers with Death's scythe. His importance is felt much more in the emotions that he brings out in Dean. Alastair at the end of everyday in Dean's stay in hell, he would offer up his blade to Dean to torture other souls. He made the same offer to his father for a longer period of time, as time is different in Hell where a month in Earth time is a year in Hell. For thirty years Dean said to stick where the sun don't shine, but he was not strong enough for his final ten years. Eventually, Dean picked up his blade and stopped himself from being tortured for eternity by Alastair. In doing so, Dean broke the first seal which jump started the Apocalypse.
Dean's time in Hell with Alastair changed him into the Dean we know today. Alastair even tells Dean that he "carved him into a new animal" and that he left some of himself back in the pit. In the episode "On the Head of A Pin", Dean is ordered by the angels to torture Alastair for information on who is killing off angels. Dean explains that he cannot do it because they will not like what comes back out through that door when he is finished. Castiel tells him that he is the most skilled torturer that they have seeing how he became Alastair's student in his final years in Hell, to which Castiel then explains to Dean that Alastair is an expert torturer. The torturing of Alastair went terribly wrong when he was let out of the Devil's Trap that Uriel, and Castiel set up. Alastair got out of his restraints and beat up Dean badly where Castiel saved him. Castiel was also nearly sent back to heaven by a spell Alastair was performing until Sam came in hopped up on demon blood and used his powers to kill him.
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Alastair's Effect on Dean
After the episode "On the Head of a Pin", Dean was different from what we are used to. We had the revelation of him stopping his torture only to bring that pain to others in hell, but seeing Alastair on Earth coupled with the other reveal of Dean bringing forth the Apocalypse it all became a little too much for Dean to handle. Even worse was the fact that he didn't know how far off the rails Sam had gone, but Alastair's presence made him begin to lose his cool and lose faith in what he believed in. He didn't believe in God, obviously, but he lost faith in his brother and in himself. One of the best lines in the series is when he said to Castiel, "I guess I'm not the man either of our fathers wanted me to be." The line shows that Dean doesn't believe that he is strong enough to carry the burden to stop the Apocalypse as he is told that he is the only one that can.