Consecration was the magical act of burying a dead witch so their soul may join the Ancestral Plane. By carrying this out, their magic flowed back into the Earth, fueling the witch community.
As seen occasionally, the process was very complicated and the body of the witch that was being consecrated had to be dressed in white. Rose petals would then be thrown over the body and it would be sprayed with what appeared to be blessed water. Upon the end of the ritual, the soul of the consecrated witch would join the Ancestors in a limbo-like dimension and their power would flow into the earth, fueling the community. With the destruction of the tether between the Ancestral Plane and the community of New Orleans — thereby eliminating Ancestral magic — consecration is no longer a viable method to fuel magical practice.
History
In 1742, all the Elders in New Orleans were massacred. In order to complete the Harvest and regain their power, the witches needed to become Elders. So, the witches decided that the first one to consecrate the most powerful witch would become an Elder.
Throughout The Originals series
In Always and Forever, Sophie was seen towards the end of the episode as she consecrated Jane-Anne's body to the earth, with her fellow witches watching the burial. Rose petals were thrown across the body and Sophie poured a few drops of blessed water across it, while crying. In Tangled Up In Blue, after reluctantly causing Katie's death, Sophie consecrates her to the earth, with the other witches watching sadly as yet another witch is dead.
In Sinners and Saints, Sophie together with Rebekah and Hayley despite Klaus' order left to go to the bayou and get the remains of the dead witches that have been killed the night before. According to Sophie if their remains are not consecrated by sundown their power will fade though this remains debatable. As seen later, Sophie didn't need the whole bodies for the consecration as she only ended up taking a few vital parts for it to work, such as the blood and hair of the deceased witches. In Après Moi, Le Déluge, after the death of their last Elder, Sophie Deveraux planned to consecrate the bones of Céleste Dubois (a powerful witch that was never consecrated) and become an Elder in order to complete the harvest and regain power for the witches. However, Céleste's power was not infused into her bones, much to Sophie's dismay. To rectify this and complete the harvest, Elijah offered up the bones of Esther, a powerful witch whom was never consecrated. Rebekah is against the idea and fears that the witches would use Esther's power against them. However, Klaus agrees with Elijah, unwilling to see the destruction of New Orleans at Davina's hands.
Elijah states that if Esther is buried on property owned by one of her descendants, she becomes a New Orleans witch. He also states that, as her family, they share the Ancestral Magic. Rebekah quickly stops Elijah and reminds him that they are vampires and cannot practice magic or own property. At the same time, Sophie explains to Sabine that since they can't practice magic, they will channel all Esther's magic to her, but the only requirement is that they need to participate in the Harvest. Elijah informs Rebekah that Hope is Esther's descendant and that now Hayley owns the land, so does Hope. Rebekah is still skeptical about consecrating one of the most powerful witches in history, as they will hand all the power to her enemies, but eventually agrees. Later on after Esther is consecrated Sophie channels all of her power and so becomes the only living Elder of the coven. All of this is enough to finally complete the Harvest though it is jacked quickly after.
In Long Way Back From Hell, Monique is seen consecrating her dead aunt Sophie when Elijah interrupts the ceremony. Monique later on suggests that while Sophie did join the Ancestors, she will be tortured due to her lack of faith in the Harvest & witchcraft overall. In Le Grand Guignol, After Elijah convinces Monique to help him. She reluctantly does so and ensures that should Celeste try to possess another witch, she would come back to her own reconstituted body. So it happens and it is later suggested that after Celeste's death Monique consecrated her to the earth to finally join the ancestors after evading death for 200 years. In A Closer Walk With Thee, Genevieve explains to her fellow witches how the ritual of consecration saves the ancestral witches from the torments of the purgatory known as the other side. Instead, the ancestral witches who are consecrated upon their deaths join the Ancestors in a limbo-like dimension where they act as a power source and fuel the earth with power for the living witches.
In I Love You, Goodbye, Rebekah reassures Kol at his deathbed that since he will die as a witch, they will consecrate him to the earth entrusting his spirit to the Ancestors. Specify then, that this kind of spirits can be brought back to life, as shown by the way in previous cases when Celeste has resurrected Papa Tunde, Genevieve and Bastianna or same as the Harvest Girls.
In You Hung the Moon, the witches of New Orleans consecrate the bodies of the twelve Versailles witches who were part of the Ninth Ward Coven, massacred and burned in a square of their neighborhood. Unlike the French Quarter Coven's consecration ritual, the Ninth Ward Coven burial ritual is slightly different: they used incenses burning instead of torches and candles, flower offerings, a red coat over the tombstone and a white shroud wrapped around the bodies instead of the black one as shown with Jane Anne's body. These has elements in common with the Vietnamese culture which is the one the Versailles witches belong to. In Dead Angels, Davina takes the opportunity to consecrate Ariane as a New Orleans witch stating that witches take care of their own. This allows her to see into her past. Ariane's spirit visited her at the end of the episode.
Incantation
The Elder witch implements a number of fumigations and aspersions on the body of the deceased witch. In the case of recent deaths, such as Jane Anne or Katie, spread also white petals and the same body is dressed in white before being buried.
In the case of old bones from which a witch must draw power (as when Sophie tries to seize power from Celeste's old bones) must use ashes and do fumigation, then take the skull and invoke the ancestors. While holding the skull over the bones of the witch, chant:
I consecrate these bones to the Earth, Ancestors hear me.
Trivia
Consecrated witches moved on to the Ancestral Plane, where they fueled the community's power.
The rite of consecration, besides ensuring to the witch's soul a passage in the Ancestral Plane, also serves as a funeral rite.
It was shown if a witch dies again they automatically return to the ancestral plane, except in Esther's case (She found peace after her final death.)
However its debatable seeing as Vincent needed to reconsecrated Davina so it's not known if she was an special case have been shunned months prior to her ultimate death.
It was decreed by the Ancestors that Hope Mikaelson be consecrated among them and allow her power to fuel the witches of New Orleans. However, due to the fake-death of Hope and the murder of Esther, this was never fulfilled.
A witch does not need to practice Ancestral Magic to be consecrated.
Their magic, however, fueled Ancestral Magic, despite the witch not practicing such whilst alive.