Overview
Alastair is a fictional character on The CW Television Network's drama and horror television series Supernatural, appearing in its fourth season. A particularly infamous demon and torturer in Hell, he is portrayed in succession by actors Mark Rolston, Andrew Wheeler, and Christopher Heyerdahl due to his demonic ability to possess human hosts.
The writers created the character to explore series protagonist Dean Winchester's experiences while in Hell, particularly Alastair's tutelage of Dean in torturing other souls. The character received generally favorable reviews from critics, with fans at the time considering him one of the series' best villains.
Plot
When Dean Winchester is sent to Hell at the end of the third season, it is the demon Alastair who tortures him, stopping only when he eventually convinces Dean to torture other souls himself. Alastair begins training Dean as one of his apprentices until fourth season premiere "Lazarus Rising," in which Dean is rescued from Hell by the angel Castiel, who requires his assistance in stopping Lilith from breaking the mystical seals on Lucifer's prison. It is subsequently revealed that while the man Alastair was possessing did get vaporized, the demon himself survived.
Alastair (Mark Rolston) makes his debut in the episode "I Know What You Did Last Summer," in which he seeks to capture and interrogate Anna Milton, a fallen angel who can still hear the conversations of other angels. Dean and his brother Sam interfere, but Sam's demonic powers are too weak at the time to harm Alastair. When they try to kill him with Ruby's demon-killing knife, they accidentally stab him, with the demon laying claim to the knife as the brothers flee the area with Anna and Ruby. Alastair is later tricked by Ruby and the Winchesters into a confrontation with the angels Castiel and Uriel in the episode "Heaven and Hell" after torturing Ruby with her own knife for Anna's location. Alastair proves to be more powerful than the angels and quickly gains the upper-hand on Castiel, but is seemingly vaporized in the blast generated by Anna being restored to her angelic true form, the only thing left behind of Alastair being the demon-killing knife.
From Wikipedia (CC BY-SA 4.0).