A curse, or hex, is a type of spell work done to cause misfortune, harm or death on whoever it is placed on.
Euchee Tribe Curse
Two hundred years ago in Oasis Plains, Oklahoma the American cavalry came to relocate the Euchee Tribe. The Euchees were resistant to being taken from their land. This caused the Calvary to become impatient and on the night "the moon and the sun share the sky as equals," the cavalry raided the Euchee village, raping and murdering the tribe. These rapes and murders would go on for six nights until every man, woman and child still in the village was dead. As an a final act of retribution, on the sixth night, the dying village chief whispered to the heavens, "that no white man would ever tarnish this land again." The valley would be protected by nature, which would bring as many days of misery and death to the white man as the Calvary had brought upon the Euchee people.
A hundred years ago, in Baton Rouge, a conjure woman created the rabbit's foot under a full moon, in a cemetery on Friday the 13th. The cursed rabbit's foot would grant the possessor exceedingly good luck, but once the foot was out of the hands of the owner, the luck would turn, eventually leading to death. The curse was eventually stopped by Sam and Dean by burning the rabbit's foot in a cemetery with bone ash and cayenne pepper.
After a young woman named Corey performed a ritual that summoned Veritas, the goddess of truth, all people in Calumet City, Illinois who asked aloud for the truth invoked her curse. Once invoked, the person is bombarded with harsh truths until they are driven to suicide as a tribute to Veritas.
The Mark of Cain was both lock and key created by God to keep his sister the Darkness imprisoned. The Mark was entrusted to Lucifer, but soon began to assert its own corrupting will, revealing itself as the first curse. Lucifer would pass the Mark onto Cain after killing his brother Abel, and Cain in turn would pass the Mark onto Dean Winchester. The Mark slowly begins to corrupt, amplifying the darkest compulsions of the bearer. If a human bears the Mark, they must continually feed it, otherwise they will succumb to the side effects of its immense power. If the bearer were to die, they would be resurrected as a demon. To remove the Mark without having passed it along to another bearer unlocks the cage that holds the Darkness, releasing it upon the world.
Kiss of Death
The "Kiss of Death" is an Aramaic curse created by the witch Sonja. Once the ritual is performed, it must be sealed with a kiss. The kiss marks the person for a qareen, who will appear as the persons deepest, darkest desire before ripping their heart out. The curse can be transferred by kiss, but once the qareen has killed the cursed individual, it will work back until it gets to the original person who performed the curse. The only way to lift the curse is by finding the qareen's heart and stabbing it.