Witnesses are characters on Sleepy Hollow. Their job is to protect the world from evil. There must be two Witnesses at a given moment so if one of them dies, the essence of their soul moves on to the next chosen Witness, suggesting that their souls are eternal. 
It's unknown how the First Witnesses, Ichabod Crane and Grace Dixon, were chosen. Their descendants weren't mentioned, except from an ancient tablet he found from his family tomb in Scotland before the events of Season 3. It was over 2000 years old and it was titled "The Legend of Sleepy Hollow." It said in Sumerian that they were known as the Destroyers. They weren't really meant on devastating the world, but it was taken out of context that there must always be two individuals, as Ezra explained. Unfortunately, Ichabod wasn't able to decipher the entire tablet before the Hidden One demolished it.
Fast forward to 1751 when Ichabod was born. Grace Dixon's birth was unknown, but both she and Ichabod were the next generation. After centuries of dormancy, they carried on the mantle; however, it was undisclosed how it was passed onto them. Then during the Revolution, General Washington created his own Bible as well as Revelations. It consists the roles of the two Witnesses, as well as a map leading to the location of Purgatory. During the defeats of Ichabod and the Horseman of Death, Katrina and Washington buried the Bible with the Freemason's body in a secluded cave in Sleepy Hollow. In Season 1, it was proven very helpful for Ichabod and his allies, especially when he proved to Abbie of what the supernatural war was about.
Then it was discovered in the Corbin files, that Abbie is discovered as the next Witness. It was soon unraveled that she belonged to a family of Witnesses. Her bloodline was unveiled as she later found out that it started with her great-aunt, Grace; then upon her death from a fire at Frederich's Manor caused by cries of Henry Parrish as a baby, it was passed onto M. Dixon. After she was petrified by the Gorgon, it was later onto her mom, Lori, who then succumbed to spiritual torture. Lastly, it was finally onto her after she committed suicide by a menacing spirit of a nurse at Tarrytown.
In a dream, Abbie saw her deceased mom in Purgatory. The place she was in front of revealed the location of the Sword of Methuselah, as well as the petrified M. Dixon. It was a connection to her role as a Witness. Ichabod came to the conclusion that the ruins were important to him because of the Horseman's quest for the sword. Before they forged it, he even said that he sees his destiny clearly thanks to Abbie.
On Season Three's premiere, I, Witness, Ichabod discovered the tablet. He was then incarcerated and with Abbie's help, now an FBI agent, he was released and it was sent to the Archives. He later attempted to decipher it, until it was unexpectedly destroyed by the Hidden One. But before that, it had some interesting effects.
Abbie's time at the Catacombs were significant as she came across a mysterious sign. After she left, she was distracted by it. Always seeing it at the most inopportune times. Even during a meeting, or when Ichabod was in peril by the New Jersey Devil's poison. It was even there at the back of her shed, and it was scribbled many times in Lori's journal. Thanks to the golden pieces shaped as the symbol of an old projector, Ichabod discovered them. With a professor, he carefully examined them with the tablet. The pieces suddenly attached themselves to an upper corner of it, and he had a vision on where Abbie was and fighting a mummy. It led him to her and the symbol known as the Emblem of Thura wasn't entirely evil. It was a sign showing the Witnesses must always become one at all costs. It was even Abbie's voice that helped Crane stop wandering in the darkness in his astral form, and wind up safely in the Archives. Then the object saved his life as it absorbed a magic spell from the Hidden One; it was rumored to be more powerful than himself, until he destroyed the tablet.
Everything led up to Ragnarok. When Abbie and Ichabod returned to the Catacombs to find answers on defeating the Sumerian god, Pandora's Box opened and a shadow enveloped her, causing her to blackout temporarily. Ichabod said that the tablet said a soul must be used to close Pandora's Box permanently. Apparently, this was a foreshadowing of her ultimate fate. When they got to Pandora and the Hidden One, minus Joe, one of their souls must be sacrificed to defeat him for good; because the shadows chose her, Abbie reluctantly accepts. Her soul and body vanished inside, trapping her.
As Ichabod and Jenny made the final attempt to defeat Pandora and return her soul, the box exploded. In a dream state Ichabod discovered that Abbie's life has come full circle. They were in a place between life and death. She assures him they're free and are eternal souls. She later said her role as a Witness is done. He wants them to end their fight together, but it's a new beginning for her and she knows her place. They say goodbye one last time.
Finally, Ezra explains to Crane at the cemetery of the Witness' soul. It moves onto another In her extended bloodline, sparking the new Witness, and the bearer happens to be in Washington, DC.
Upon the event of Abbie's demise, a young girl named Molly had a fainting spell at school. Since then, she's been sketching in her notebook with a lot of drawings of Ichabod. The doctors said it was transient aphasia, which disables her ability to talk. However, after she saw Ichabod in person she was cured instantly. It turned out that her soul merged with Abbie's after death.
Meanwhile, Ichabod discovered her through a dream at the Archives. It linked him subconsciously to Molly, whom he didn't know yet. On a photo of her from Diana's phone, he put two and two together. It was then when he encountered her for the first time. She was able to save Ichabod's life since she was the only one to reverse the curse from the Spider Demon, saying he's a bringer of hope and he should never give up on hope. Because she was his anchor, she was successful and she used Abbie's last words to save him. Molly eventually turned 11; according to Washington's Bible, the Witness is granted his/her destiny. It was unclear why Molly was chosen though.
Finally, there was her future counterpart, Lara, She traveled into the past to prevent Malcolm Dreyfuss on achieving his goals. She turned out to be Molly's future vessel who once admired him. When Ichabod became the Horseman of War, she managed to use a decoupling spell to separate him from it. As she was trying to make him remember who and what he was, she succeeded as he fought the avatar away from his mind. After that, Dreyfuss went to Hell from Jobe and the Philosopher's Stone, and Molly had the freedom to choose whether or not she wants to be a Witness. Having Lara around was an example that the Witness' mantle can be passed on without dying. Molly's decision was never mentioned.
The current Witnesses are Ichabod Crane and Molly Thomas along with her future self, Lara Thomas.
- There must always be two Witnesses active at any given point. 
- As a soul of a Witness is eternal, once they die, the essence of their soul moves on to the next Witness.  For example, upon Abbie Mills's death, the essence of her soul moved on to the next chosen Witness which is someone from their extended bloodline, Molly Thomas. 
- When a Witness reaches the age of 11, they are granted with the responsibility and power of a Witness. 
- Like other humans, Witnesses may be made into Horsemen. The creation of a Horseman from a Witness does not trigger the calling of the next Witness. 
- Only two Witnesses can exist at any one time. When Lara, a future version of Molly, traveled through time, she inherited her mantle as a Witness from her, making Molly the first Witness who had their mantle passed onto another person without dying. 
|"Pilot"||"Blood Moon"||"For the Triumph of Evil..."||"The Lesser Key of Solomon"||"John Doe"|
|"The Sin Eater"||"The Midnight Ride"||"Necromancer"||"Sanctuary"||"The Golem"|
|"The Vessel"||"The Indispensable Man"||"Bad Blood"|
|"Columbia"||"In Plain Sight"||"Heads of State"||"The People vs. Ichabod Crane"||"Blood from a Stone"|
|"Homecoming"||"Loco Parentis"||"Sick Burn"||"Child's Play"||"Insatiable"|
|"The Way of the Gun"||"Tomorrow"||"Freedom"|
|Bones: Season Eleven|
|"The Resurrection in the Remains"|