The mark of the witnesses.
The Rising of the Witnesses is one of the 66 Seals that was broken by Lilith. Forcing ghosts of people who witnessed the supernatural to rise. She used ghosts of people hunters could not save going after the hunters and killing them. Carl Bates, R.C. Adams, Olivia Lowry, and a hunter named Jed all die when the Witnesses rise and kill them, pushing their arms into the hunter's bodies exploding their chests from the inside out.
After discovering the bodies of the dead hunters Bobby heads home and Sam & Dean soon follow. Bobby is eventually attacked by the two girls who he could not save when he first started hunting and taken to the Salvage Yard. As Sam and Dean arrive Dean at Bobby's place Dean is attacked by the ghost of Meg and Sam saves Bobby in the Salvage Yard. Dean, Sam and Bobby soon regroup and head to Bobby's Panic Room to prepare shotgun shells and defenses against the Ghosts. Bobby figures out this is a spell called the Rising of the Witnesses - Ghosts of the deceased that witnessed and were killed by Supernatural means. Sam, Dean and Bobby lay the Witnesses to rest with a spell.
Bobby, Dean and Sam head to the living room spreading salt lines and to prepare the spell. One which will hopefully put the Witnesses to rest. Some of the ingredients are opium, wormwood, hemlock and a sigil drawn on Bobby's desk. The Witnesses, including Victor Henriksen, Ronald Reznick, Meg Masters and the two young girls Bobby failed to save attack, spreading the salt lines as Dean and Sam protect Bobby using the shotguns filled with rock salt. As the Ghosts close in the spell is nearly completed as Meg traps Sam behind a desk, Ronald and Victor attack Dean and Meg moves onto Bobby to kill him. The spell is soon completed with Dean throwing in the crushed ingredients into the fireplace as a blue white light engulfs Bobby and spreads to the entire room putting the Witnesses to rest.
Castiel visits Dean in the middle of the night mentioning that although this seal was broken that there are many others still out there.