Alastair was a powerful, white-eyed demon who was Hell's grand torturer, and Meg apprenticed under him in the field of torture. He was able to travel from Hell and Earth with incredible ease, something he shares with Azazel. His stature to Lilith is vague, but the way he grudgingly followed her orders indicates that she at the very least out-ranked him. Dean Winchester first encountered Alastair in Hell, and he was the one in charge of torturing him on orders directly from Lilith, as to break the first seal.
During the 40 years (or 4 months in Earth time) that Dean was in Hell, each day 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.
Powers and Stats
Tier: At least Low 7-B
Name: Alastair, Hell's Grand Torturer, Picasso with a Razor
Age: At least Thousands of Years Old
Classification: White Eyed Demon
Powers and Abilities: Superhuman Physical Characteristics, Immortality (Types 1 and 3), Regeneration (Low-Mid), Telekinesis, Soul Manipulation, Possession, Immunity to Divine Abilities (Smiting), Teleportation, Biokinesis, Telepathy, Flight, Non-Corporeal in True Form, Magic User
Attack Potency: At least Small City level (Easily overpowered base Castiel)
Speed: Unknown. At least Subsonic+ with Supersonic reactions
Lifting Strength: Unknown, likely at least Superhuman
Striking Strength: At least Small City Class
Durability: At least Building level+, possibly Small City level in Host. His true form's nature as a Non-Corporeal spirit makes him hard to kill.
Standard Equipment: None
Intelligence: Should be quite High.
Weaknesses: Devil Trap, Hex Bags, Special Children, Holy Water, Rock Salt