Rise of the Witnesses
The Rise of the Witnesses is a sign of the Apocalypse. It is one of the 66 Seals that, when broken, will unlock and open the "door" that traps Lucifer in his cage. The Witnesses are people who have seen, or died at the hands of, supernatural beings. They can be distinguished from normal ghosts by a symbol branded on them. Once summoned by the proper spells, they act in a manner similar to other vengeful spirits, but target a specific person with their wrath. They will kill the individual if they can. Once identified, they can be banished by a spell. According to Bobby Singer, its not their fault that they act this way: they were forced to rise and are like animals more than sane spirits.
Lilith breaks the seal, and the ghosts of people that hunters failed to save return to kill them. Hunters who are killed include Olivia Lowry, Carl Bates, Jed and R.C. Adams. According to Castiel, 20 hunters were killed in total.
Bobby Singer is attacked by two young girls; Sam and Dean are attacked by Meg Masters, Special Agent Victor Henriksen, and Ronald Resnick. While the Winchesters hold the spirits off, Bobby starts a spell to put them back to rest. Meg nearly kills him, but he has done most of the work and Dean is able to finish the spell. With the spell cast, all the Witnesses are put back to rest.