Horcruxes
Christoph: He saved his life. Well, he saved his soul. Like, literally. The watch, it's like a -- it's like a Horcrux.
Sam: It's a Harry Potter thing.
Dean: Oh, you would know that.
Sam: Anyways...so the watch holds Hitler's soul?
Ellie: Right. You -- you trapped Hitler's soul in a pocket watch? Why is that? For safekeeping?
Christoph: (Scoffs) No. To bring him back.– Sam, Dean, Christoph Nauhaus, and Ellie Grant, 12.05 The One You've Been Waiting For
History
Horcruxes are objects that contain the Soul of a person. Before a person dies, they can place their soul inside an object to remain alive if their body is destroyed afterwards. The Horcrux can take a Vessel inside another person if that person contains the blood from the family line of the person whose soul will inhabit the body. It will dig into the vessel, deactivate the soul that is already there, and reawaken the vessel as the reincarnation of the person who will take control of the body.
12.05 The One You've Been Waiting For
Before he died, Adolf Hitler agreed to have his soul preserved within his Pocket Watch by Commandant Nauhaus, a high ranking member of The Thule Society. Thule agents were attempting to move the pocket watch to another location until it can be used to revive Hitler when they were ambushed and killed by the Soviets, losing the watch to them in the process. The watch ended up with the Russian family of the Soviet agent who kept the watch as a memento. It bounced around to China and Peru and multiple other places around the world until it ended up at an antique shop at Colombus, Ohio in 2016. The owner and a woman who wanted to purchase the watch were both murdered through Spontaneous Combustion by another Thule agent.
Sam and Dean were able to track down a Thule Society member named Christoph Nauhaus who revealed to them their plan to resurrect Hitler, including the fact that the Thule Society keeps track of every one of Hitler's descendants, including Ellie Grant. Two Thule Society members went to retrieve Christoph and Ellie to continue their plans, but were called back as they were battling against the Winchesters after retrieving Christoph. Ellie, who escaped, was later abducted by Commandant Nauhaus.
Commandant Nauhaus infused Ellie's blood into his body to be used as a vessel for Hitler. The pocket watch was placed onto his chest where it opened the Commandant's chest with four small blades built within the design of the pocket watch. The scar it left behind after it went inside the Commandant's body resembled a Nazi Swastika. Hitler had an exhilarated and very awkward reunion with the other Thule Society members who were present with him during World War II who were currently present during the reincarnation. Sam and Dean came in to take out the Thule Society members and save Ellie after Christoph, who was sentenced to death by his father; the Commandant, showed them where they are after he escaped. While Hitler was distracted by Sam and Dean, Ellie was able to break free from her restraints and shoot one of the Thule agents. Sam and dean were able to kill the other remaining members. Dean was able to restrain Hitler and penultimately said "Heil this." before shooting Hitler in the head, which makes Dean very proud of himself. It can be assumed that the pocket watch was destroyed with Commandant Nauhaus when Sam and Dean incinerated the bodies.
Trivia
The term "Horcrux" was used in the last three movies of the Harry Potter Movie Franchise to describe objects that contain parts of the soul of a wizard to render them immortal until they are destroyed by Basilisk Venom.
