Samhain

Samhain, also known as the origin of Halloween, is a powerful and special demon of Hell and is one of the 66 Seals. He can only rise when summoned by two powerful witches through three blood sacrifices over three days, that last day on the final harvest, Halloween. Once Samhain has risen, he is able to raise ghosts, zombies, ghouls and unleash them onto the Earth.

Early Life
Samhain is one of the oldest demons in existence being created around the time of the Knights of Hell. When God made the archangel Lucifer's cage, he made the summoning of Samhain one of the sixty six seals. He was worshiped as a god on Halloween by the ancient Celts. When he reigned on Earth on Halloween night, people kept their children indoors. They wore masks to hide from him, carved pumpkins to worship him, and left sweets at their doors to appease him. He was exorcised centuries ago by an unknown party. As he can only be summoned once every 600 years, he was trapped in Hell.

Season 4
Samhain, once summoned by Tracy Davis and Don Harding on Halloween 2008, possesses the dead body of Don Harding. When he enters the body, the corpse's irises turn either grey or pale blue, while his pupils stay black and the remainder of his eyes stay white, marking him as a different kind of demon than any that has appeared on the show before. His unique eye color may mean that he is somewhere between the white eyed demons, such as Lilith, and the common black eyed demons.

Samhain proceeded to kill Tracy and then go on a rampage, entering a mausoleum in a cemetery and releasing ghosts and zombies to kill Don's students. While Dean Winchester dealt with his monster minions, Sam Winchester faced him armed with the Demon-Killing Knife. While Samhain's White Light had no effect on Sam, he proved too powerful for Sam to beat with the knife and Sam was forced to use his psychic powers to exorcise Samhain. Samhain proved to be powerful enough to resist Sam's powers, but ultimately he was exorcised and sent back to Hell by Sam. However, while Samhain was exorcised, his rising broke one of the 66 Seals.

Personality
Samhain was shown to be cruel and sadistic, he took pleasure in trapping his host's students and summoning zombies to devour them, so much so, that he playfully rubbed his hand against the gate as he walked off. However, he seemed more interested in simply creating and causing destruction, rather than running it, as rather than consolidating his efforts or powers, he simply passed through the area, causing chaos.

He also showed no loyalty, casually killing Tracy Davis, despite the witch being the one who loved and freed him. He was also shown to be treacherous and cunning, and had a very violent and ferocious side to his personality. Although willing to use his powers to dispatch attackers, unlike most demons, he was also more than willing to enter physical combat, and try to catch and rip them apart limb from limb with his bare hands.

He was also shown to be quiet, and did not talk much, also when angry, he would start snarling like a wild animal.

Powers and abilities
Samhain is an incredibly powerful demon who displayed multiple rare abilities as well as abilities unique to himself. That his rise was one of the 66 Seals marks Samhain as a special demon; the only other demon who was a seal was Lilith. His rank in the hierarchy of Hell is unknown.
 * Advanced Summoning - Samhain has the ability to summon creatures without the need of an external power source. Sam also stated that the longer Samhain is free, the more dangerous monsters he would summon, until everything the Winchesters had ever fought would come to him; this would include ghosts, zombies, other demons, witches, monsters, hellhounds, and conceivably more.
 * Necromancy - Samhain has the power to summon ghosts and bend them to his will, as shown when he had one attack Dean.
 * Reanimation - Similarly, Samhain has the power to turn nearby corpses into zombies and bend them to his will, as shown when he had them attack a group of party-goers.
 * Advanced Immunity - Being one of the most powerful and ancient demons, he has an great amount of immunity similar to the White-eyed Demons. This ancient demon is immune to iron, as he was able to close an iron gate and ran his hand over it as he walked away without getting burnt. He was also able to resist Sam's powers better than other demons. He cannon Demon-Killing Knife, it only caused mild pain when Sam stabbed him with it.
 * Immortality - Although Samhain's exact age was not stated, he is at least over six hundred years old and it is implied that he is thousands of years old. The fact that his rising was one of the 66 seals to free Lucifer, indicates he was around by the time Lucifer's cage was created.
 * Invulnerability - Samhain cannot be killed by conventional methods, weapons.
 * Possession - Samhain used the corpse of one of his deceased witch followers as his vessel.
 * Super Strength - Samhain was able to throw Sam across the crypt with great force and minimal effort. He was also able to effortlessly break Tracy's neck with his bare hands.
 * Astral perception - He could perceive the true form of Tracy Davis, behind her young appearance.
 * White Light - Samhain has the power to emit a fiery blast of white light that could destroy anything in its path. He was one of only two demons known to have the ability.

Harming, Misleading or Trapping

 * Exorcism - For being a more powerful demon being exorcised has a greater affect on him as he can only be summoned once every 600 years.
 * Partial Blindness - Unusually for a demon, Samhain's eyesight is impaired, causing him to see the world blurred.
 * Demon-Killing Knife - This knife could only cause Samhain pain.

Banishing and Killing

 * Special Children - Samhain was vulnerable to a special child's powers and he was exorcised by one.

Appearances

 * Season 4
 * It's the Great Pumpkin, Sam Winchester
 * I Know What You Did Last Summer

Trivia
Samhain
 * Despite the students wearing costumes in the mausoleum, Samhain was able to see them, even though he couldn't see Sam or Dean properly when they were covered in blood. However, some of them were not wearing masks, which is identified as the key component for tricking him, possibly revealing their humanity to him.
 * In reality, Samhain is the Wiccan holiday on Halloween where the Wiccans celebrate their dead ancestors, loosely based upon an older Celtic holiday of similar name.
 * The ancient Gaelic holiday of Samhain, historically celebrated in Ireland and Scotland, actually fell on November 1st, not October 31st, and signaled the start of winter. It was one of the four main Gaelic holidays, each marking the beginning of a new season. The others were Imbolc (February 1st), Beltane (May 1st) and Lughnasa (August 1st). It was traditionally believed that on Samhain, otherworldly spirits would enter this world and blight all crops and green vegetation with their breath Link
 * The festival of Samhuinn is at the end of the year for the Celts, when the Otherworld and normal world were able to be crossed. It represented ending the year with darkness, and beginning the year not too long after, which corresponds with some of the character's thoughts and actions in this episode.
 * In this episode the name 'Samhain' was pronounced Sam-Hane, its correct pronunciation is Sow-ann
 * The Celtic festival of Samhuinn is celebrated in the myth the Kelpie, in which souls are brought back from the dead, supporting Samhain's powers.
 * In Bobby Singer's Guide to Hunting, Samhain was described as a blue-eyed demon. On very close inspection, it can be spotted that his irises are a very pale blue or possible grey.
 * That Samhain's raising was one of the 66 Seals indicates that Samhain already existed by the time Lucifer had been trapped in his cage. The only other demons confirmed to have been made by that time were Lilith, Cain, Abaddon and the Princes of Hell.
 * Samhain's White Light ability was the second trait he shared with Lilith, the first one being part of the 66 seals.
 * Samhain along with Ramiel, are the only high-tier demons that appeared in just one episode.
 * Samhain's eyes are similar to those of the Rakshasas.