Alastair

Alastair is a white-eyed demon who, seemingly, is able to travel from Hell to Earth and is of the same stature as Lilith. Dean first encounters Alastair in Hell and is the one in charge of torturing him. Each day, for 40 years, Alastair would ask Dean if he wanted to trade his place for another soul. For 30 years, he refused and, each day, was chopped and killed and then placed back together, only for the cycle to continue. He couldn't bear the pain much longer and, for 10 years, tortured souls. This makes Alastair directly responsible for triggering the Apocalypse.

Alastair resurfaces on Earth to try to capture Anna Milton and use her gift for their gain. Although his initial attempt was unsuccessful, he manages to get a hold of Ruby's magical knife.

Later, he returns and strikes a deal with Ruby, who promises Anna for the safety of the Winchesters and herself. Alastair, after torturing Ruby, accepts. Unknowingly, however, he falls into a trap set by the Winchesters and later vanishes after Anna regains her grace.

He returned a short while later, possessing a new body. He was behind the abduction of a reaper in a small town. He encountered Sam and Dean and fought them, before escaping when Sam revealed that he's immune to his power. He gained a new one soon after and abducted a second Reaper named Tessa who Dean knew. He planned to kill the two reapers on the night of the solstice moon to break another of the 66 Seals that needed to break in order to free Lucifer. He failed, only being able to kill one reaper, and was captured by the angels.

After seven angels were killed, Castiel recruited Dean to torture information out of Alastair. Alastair simply mocked the fact that his former protege would be the one to try and break him. Alastair revealed that the first seal was broken when Dean gave in in Hell and began torturing people. This explained why the angels had rescued Dean from Hell. Alastair managed to break free from the Devil's Trap he was binded in through water dripping from a pipe. He quickly beat up Dean and nearly banished Castiel to Heaven until Sam arrived and pinned Alastair against a wall using telekinesis. He tortured Alastair until the demon revealed that the demons were not behind the angel killings. Alastair then challenged Sam to excorcise him but Sam was now more advanced with his powers and simply used his abilities to kill Alastair in a similar fashion to what the Colt or Ruby's Knife would do. It was later shown that Uriel was behind the angel killings.

Family
Lucifer (Creator)

Demons (brothers and sisters)

Abilities
Expert at torturing 

Magical Knowledge (Rituals)

Master Manipulator (Alastair manipulated Dean Winchester into torturing other souls)

Powers
Alastair is a white-eyed demon like Lilith & has been shown to be stronger than most demons as he could partially resist Ruby's demon-killing knife, the powers of low ranking angels and able to survive an encounter with Anna's angelic explosion. He has been seen able to possess people like most demons & have enhanced strength, speed, stamina, durability, & telekinesis. Alastair was originally immune to Sam's powers during their first encounter. Alastair apparently could send angels back to Heaven as he tried & almost succeeded to do so to Castiel twice. Demons are immune to all human diseases.

Super strength 

Invulnerability 

Enhanced speed 

Super endurance and stamina

Telekinesis 

Demon title
The Head Torturer and Chief Executioner of Hell.

Season 4

 * "I Know What You Did Last Summer"
 * "Heaven and Hell"
 * "Death Takes a Holiday"
 * "On the Head of a Pin"
 * "When the Levee Breaks" appears as hallucination and tortures Sam
 * Alastair was also present while Dean was in Hell.