A hex bag is a supernatural component commonly used by witches as a means to cast a hex, which injures or kills people who they seek vengeance against. It made its first appearance in Malleus Maleficarum, after the Winchesters find them hidden behind cabinets and under sofas.
They are usually filled with substances used in witchcraft such as herbs, black cat bones, and even the charred bones of newborn babies. It can be made of other things as well.
When a hex bag is placed inside a room and once the potential victim is present, the person will die if intended. Once a person is being subdued, the only way to break the spell is by finding the bag and burning it.
They can also be used as means of hiding from demons, and angels as well. However, it is completely ineffective against archangels in vessels. Ruby gave Sam and Dean hex bags at the end of Jus In Bello to throw Lilith off their trail for the time being. In Sympathy for the Devil, Sam reveals that Ruby taught him to make hex bags to hide him and Dean from demons and angels. They can also be used in spells like the Dream Manipulation Spell and the Servitude Spell.