A Tulpa is a "physical materialization of a thought, resulting in the creation of a being or object." (Pad of Definitions, Official Website)
A Tulpa can be created by many people concentrating on one thing while looking at a Tibetan Spirit Sigil. Once created, a Tulpa takes on a life of its own and no longer needs people to believe in it.
John Winchester's JournalEdit
There are a few notes about the Tulpas creation and meaning:
1993 May 15 Edit
Tulpas created through intense ritual visualizations are known as dubthab. Variation known as dragpoi dubthab is specifically aimed at creating a thought-form with the idea of harming another person. The physical form of tulpas becomes apparent to the senses after the mind can begin to sense its spirit presence.
Tulpas that are created, no matter the creator’s intent, will gradually turn on the creator. Evans-Wentz wrote that enlightened magicians can destroy tulpas as easily as they can create them; also that those masters can incorporate their spiritual being into the body of another person. It’s about the willpower.
A Tulpa is unknowingly created by Craig Thursten and his cousin Dana, who paint several symbols from her theology textbook on the inside of an abandoned house, and invented the legend of Mordechai Murdoch, unaware that one of the symbols is a Tibetan meditation sigil. The legend soon spread until Harry Spangler and Ed Zeddmore put the sigil on their website, and the resulting crowdsource effect incidentally brought the Tulpa to life. The Tulpa first took on the image of Dana being hung from the rafters when a trio of teenagers go in on a dare, but when they reported to the police, the Tulpa had vanished. This attracted the attention of Sam and Dean Winchester, who investigated the house, at the same time the Hell Hounds did. Both parties found nothing and left. Soon after, a teenage brunette goes into the house on a dare, only for Mordechai to strangle her with a hang rope. The Winchesters later figure out Mordechai's true nature as a Tulpa, his M.O. constantly changing as new legends were posted on HellHoundsLair.com, so they provided their own legend that Mordechai was vulnerable to iron rounds, but the website's server crashed before the legend could take effect. Therefore, Dean set the house on fire, knowing that Mordechai could not leave the house and would burn along with it and that no one would go in anymore.
A Tulpa was thought to be behind a drama teacher and student's disappearance when they threatened to cancel Supernatural: The Musical. Dean and the play's creator, Marie, assumed that her scarecrow prop was a Tulpa and burned it in the furnace, only to later learn from Sam that Calliope was the culprit.