|Dates|| ???? – 1408 (exorcised)|
2008 (exorcised by Sam Winchester)
|Occupation|| Demon Mutation|
The Lord of the Dead
|Episode(s)||4.07 It's the Great Pumpkin, Sam Winchester|
Dean, Samhain is the damn origin of Halloween. The Celts believe that October 31st was the one night of the year when the veil was the thinnest between the living and the dead, and it was Samhain’s night. I mean, masks were put on to hide from him, sweets left on doorsteps to appease him, faces carved into pumpkins to worship him. He was exorcised centuries ago.
Samhain is a special type of demon or more a "demon mutation". When he reigned on Earth on Halloween night, people kept their children inside and wore masks to hide from him, carved pumpkins to worship him, and left sweets at their doors to appease him. In this way, Samhain was the origin of modern-day Halloween. He was known to have had a pair of brother and sister witches serving him; these witches would later be known by the aliases Don Harding and Tracy Davis. At the time, Samhain took Tracy for a lover as well as a servant. In 1408, Samhain was exorcised by an unknown force.
Samhain can be summoned out of the confines of Hell by two witches through three human sacrifices over three days, the last of which must occur on the final day of Celtic harvest, the 31st of October, the American Halloween. This ritual can only be attempted every 600 years. As such, Samhain's servants had to wait and prepare until the year 2008 before they could attempt to raise him again. They succeeded, but Don was killed for the third sacrifice and Tracy was murdered by Samhain himself, with Samhain using Don's corpse as his host.
Samhain's rise is one of the 66 Seals holding Lucifer's Cage. The summoning of the demon in 2008, during the time the seals were being broken by Lilith's forces, broke this seal despite the efforts of the hunters Sam and Dean Winchester as well as the angels Castiel and Uriel.
Samhain has pale gray (or possibly blue) irises, black pupils, and white sclerae.. His unique eye color is possibly be due to his poor eyesight. He could be classified as a third white-eyed demon as Anna Milton's drawing of him possessing Don Harding shows him displaying white eyes. Howsoever, he's classified as a demon mutation together with the Acheri demons, the Seven Deadly Sins and also the Croatoan Virus.
Powers and abilities
- Demonic energy – Samhain can emanate a demonic blast of white energy from his palms that can decimate almost any target.
- Necromancy – As the Lord of the Dead once on Earth, Samhain can raise supernatural creatures such as ghosts and zombies at will, without the use of any spells.
- Superhuman strength and durability – Samhain strength power far exceed that of regular demons. He was shown to be resistant to iron as well as the demon-killing knife when Sam slices his arm. Sam noted that the whitish-eyed demon was "pretty powerful", hinting that Samhain may have been truly immune to it's killing power. 
- Demon-killing knife – While the knife was able to cause Samhain great pain, it is unknown if it could have killed him. As Sam himself said "Might take more than the usual weapons" implies that Samhain maybe truly immune to Ruby's knife.
- Exorcism – Like all demons, Samhain can be exorcised back to Hell. While in Hell, the ritual to raise Samhain can only be attempted once every 600 years.
- Poor Eyesight – Unusual for a demon, Samhain's eyesight is incredibly poor. In this way, he can be tricked by obstructions to people's faces (e.g. blood, a mask) into believing them to be dead or non-human, and leaving them alone.
- Special Children – Sam Winchester was shown to be immune to Samhain's white light attack and was able to exorcise Samhain with his powers even when low on demon blood, albeit with great concentration and difficulty.
Wanting to summon Samhain, two witches named Tracy Davis and Don Harding (actually centuries-old siblings) start performing the sacrifices needed to summon Samhain. This draws the attention of the Winchesters and the angels as the raising of Samhain is one of the 66 Seals. Sam and Dean kill Don, but Tracy reveals herself and successfully summons Samhain, breaking the seal. Samhain possesses Don's body and recognizes Tracy before killing her. Samhain is unable to tell that Sam and Dean are alive due to a "mask" of blood they smeared on their faces so he leaves them alone.
Samhain travels to a mausoleum where a bunch of high school students are having a party, locks them in and summons zombies and at least one ghost that attack. Sam and Dean rescue most of the kids and Dean deals with the monsters while Sam faces Samhain. Sam proves immune to Samhain's power so they fight hand to hand, but even with the demon-killing knife, Sam is no match for the demon. Finally, in the end, Sam just barely manages to exorcise Samhain with his powers and sends him back to Hell.
A drawing of Samhain can be seen in Anna Milton's journal. The description says "Samhain. The next seal is broken." In Anna's drawing, Samhain's eyes are completely white, implying that Samhain may be classified as a third white eyed demon.
Samhain in Lore
Samhain - properly pronounced "sa-win" as opposed to how the demon's name is pronounced in the show (which is as spelled). It is the name of a Celtic festival that originated as an end of summer and harvest festival and was named after the Celtic name for the month. It later became associated with the Christian All Souls' Day. Some of the rituals associated with Samhain are now seen in Halloween traditions.
- Samhain may be classified as a third white-eyed demon, possessing similar unique abilities, but unlike Lilith and Alastair, his eyes have pale gray irises and black pupils, possibly due to his poor eyesight. Howsoever, Samhain self is etablished to be the Lord of the Dead and an demon mutation alongside the Acheri demons, the Seven Deadly Sins, the Croatoan Virus. 
- Tracy Davis's complaint about her experiences when waiting 600 years to summon her master indicates that Samhain was exorcised in the year 1408. 1+4+0+8 also adds up to the number 13, which has a number of meanings but is most notorious as an unlucky number to the superstitious.