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 revealed that the Witnesses, Ichabod Crane and Abigail Mills, inherited the Witness mantle from their extended bloodlines. Their descendants weren't mentioned, except from an ancient tablet Crane found from his family tomb in Scotland between the events of Season 2 and Season 3. It was over 2000 years old and it was titled "The Legend of Sleepy Hollow." It was written in ancient Sumerian and it explained how they were known as the Destroyers. They weren't really meant on devastating the world, but to destroy the monsters they face. There must always be two Witnesses, as Ezra explained. Unfortunately, Ichabod wasn't able to decipher the entire tablet before the Hidden One destroyed 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 through their bloodlines. Then during the Revolution, General Washington created his own Bible as well as Revelations. It consists of valuable information concerning the roles of the two Witnesses, as well as a set of instructions on how to locate 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 August Corbin's files, that Abbie is 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-great-great-aunt, Grace Dixon; then upon her death from a fire at Frederick's Manor caused by cries of Henry Parrish as a baby, it was passed onto Martha Dixon. After she was petrified by the Gorgon, it was eventually passed onto her mom, Lori, who then succumbed to spiritual torture by Moloch's demons. Lastly, it was finally passed onto Abbie after her mother committed suicide by Gina Lambert who haunted Tarrytown Psychiatric.
In a dream, Abbie saw her deceased mom in Purgatory. The ruins she was standing in front of was revealed to be the location of the Sword of Methuselah, as well as the petrified Martha Dixon. The experience 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 connection with himself. Before they found it, he even said that he sees his destiny clearly thanks to Abbie.
On Season Three's premiere, I, Witness, Ichabod discovered the Tablet of Witnesses. He was then incarcerated by the ICE 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 when it was used with the Emblem of Thura.
Abbie's time at the Catacombs were significant as she came across a symbol. After she left, she was distracted by it. Always seeing it at the most inopportune times. During a meeting, when Ichabod was poisoned by the New Jersey Devil, it was drawn at the back of her shed, and it was even scribbled many times in her journal. Because of a bronze pendant with the same shape of the symbol, Ichabod discovered the significance of the symbol after noticing its connection with Abbie and the Catacombs. With the help of a professor in Rochester, he carefully examined them with the tablet. The pieces suddenly attached themselves to the lower corners of the two pieces of the tablet, and he had a vision on where Abbie was fighting the Verslinder. With some help from of the Blavatsky Map, it led him to her and they soon discovered that the symbol which was known as the Emblem of Thura wasn't a tool for evil. It was a symbol representing the everlasting connection between the two Witnesses. It was even Abbie's voice that helped Crane get his astral form pulled back from the Catacombs and into his body at the Archives. The Emblem of Thura saved his life as it absorbed a magic attack from the Hidden One. It was rumored to be more powerful than the Hidden One himself, until he destroyed it along with 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 mentioned that the eternal soul of a Witness must be used to restore Pandora's Box, which had foreshadowed her ultimate fate. When they got to the Hidden One's lair, Pandora told them that the Eternal Soul of a Witness must be sacrificed to be used as a binding agent to hold the darkness of the box together. Because the Hidden One was regaining his strength, Abbie reluctantly accepts. Her soul and body vanished inside, and the box was able to completely drain the Hidden One of his power before he was killed by Jenny.
As Ichabod and Jenny made the final attempt to defeat Pandora and return her soul, she told them that she was never inside the box and that Abbie is dead before causing the box to explode right before she dies. In a dream state Ichabod discovered that Abbie's life has come full circle. They were in Limbo; a waiting room between life and death. She assures him that she is free and explains how their souls are eternal. She later said her role as a Witness is done since it was to push Crane on his way to fulfill his destiny. He wants them to end their fight together, but she knows that it's a new beginning for her. They say goodbye one last time after she tells Crane to kill some monsters for her.
Finally, Ezra explains to Crane at the cemetery where Abbie is buried. When a Witness dies, the essence of their soul moves onto another person from her extended bloodline, sparking the new Witness, and the bearer happens to be in Washington DC where Ichabod was taken there by Jack Walters.
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 was infused with the essence of Abbie's soul after death.
Meanwhile, Ichabod discovered her through a dream of the Archives induced by a Tibetan Singing Bowl. It linked him subconsciously to Molly for a short period of time. On a photo of her from Diana's phone, he put two and two together. It was at her school when he encountered her for the first time, which instantly restored her ability to talk. She was able to save Ichabod's life since she was the only one to reverse the effects of the Despair Creature, saying he's a bringer of hope and he should never give up on hope. Because she was his anchor, she was successful at bringing Crane back while she unknowingly used Abbie's last words to save him. Molly eventually turned 11 in Loco Parentis which, according to Washington's Bible, the Witness is granted his/her destiny.
Finally, there was her future counterpart, Lara, She traveled into the past to prevent Malcolm Dreyfuss from achieving his goals. She turned out to be a future version of Molly who once admired him as a father after the supposed death of Diana. When Ichabod became the Horseman of War, she managed to use a decoupling spell created by Katrina Crane to separate him from the Avatar of War. As she was trying to make him remember who and what he was, she succeeded in freeing Crane as he separated his soul from the Avatar of War. After battling against the horsemen and confronting Malcolm one last time, he died and went to Hell because of Jobe and the Philosopher's Stone fragment. Since Lara was a future version of Molly, the Witness' mantle had been passed onto Lara without Molly dying. Molly had the freedom to choose whether or not she wants to pursue the supernatural, but Molly's decision about facing the supernatural was never mentioned.
The current Witnesses are Ichabod Crane and 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"|