Necromancy is the darkest form of magic and the most dangerous of all its practices, used to bring the dead back to life. According to Gaius, even in the days of the Old Religion, such practices were viewed with the utmost suspicion, though this claim may be ironic since the manipulation of life and death was the main principle of the Old Religion. There are several ways to perform necromancy.
Nimueh, a High Priestess of the Old Religion, brought the slain Tristan de Bois back to life as a zombie-like wraith. The spell she used was: "Gehíere mé wan cniht áwæce. Beo strangra ond steacra for brecþ wáne. Uprærest wærc, Uther Pendragon!" The wraith could not be slain by normal weapons, only enchanted ones, such as Excalibur. A wraith can only be created by using dark magic to harness the grief and rage of a tormented soul and returning them to a corpse like but indestructible body. No mortal blade can kill a wraith and mortal magic is also useless against them for they are already dead. They are also said to disappear when their task is completed (Excalibur).
Over three hundred years ago this group of seven knights was seduced by a sorceress’ call. One by one they succumbed to her power, becoming a terrifying and brutal force that rode through the lands leaving death and destruction in their wake. They could not be killed by mortal weapons or by magic. They were only stopped when the sorceress herself was killed, as without her power to animate them, they became lifeless. Morgause animated the Knights once more to attack Camelot with the spell "Cnihtas Medhires, éower sáwla sind min sáwol. Onwic and cóm hér eft. Rid eft ond forsliehð eft. Gehðu, Uther Pendragon" (The Fires of Idirsholas).
When Morgause grants Arthur a wish, she offers to show him his mother. In her ruined castle, after having lit several candles on an altar, Morgause chants a spell that allows her to stop the time and Arthur is able to talk to Ygraine. The spell doesn't hold long and the doorway is quickly closed, never to be opened again. The spell that Morgause said was: "Arise mid min miclan mihte þín suna to helpe. Hider eft funde on þisse ne middangeard þín suna wæs" ("...þæt þu þonne wile great þone þe on þissum lænum lif þe geholian wæs" in Merlin - Secrets & Magic) (The Sins of the Father).
Using the Rowan Staff, carved from the Rowan Tree on the Isle of the Blessed given to her by Morgause, Morgana was able to bring life to the dead of Camelot and use them to fight the Knights of Camelot as they fought the armies of Cenred in the Great Battle for Camelot. Although Morgana was not an experienced Witch, the Staff's power channeled her own fledgling magic. The undead army were formed of animated skeletons and could be reduced to piles of bones, though these bones would reassemble and continue to fight. After Merlin destroyed the staff, the undead army were reduced to dust (The Tears of Uther Pendragon). Already being dead, the army of skeletons were unable to die until the spell was broken or the creator of the spell was killed.
A shade is a resurrected soul pulled from its rest by a necromancer. The shade possesses its former body and skills and recalls their original name, but it is in essence no more than a mere shadow of their mortal self. They are also under the necromancer’s power and may be moulded to their will, with this control lasting until the shade is once again killed and the soul released.
Morgana, High Priestess of the Triple Goddess, sought the advice of the Dochraid to raise a shade with a magical coin given her by her sister Morgause. The Dochraid told her to travel to the last of the five Gateways that separate the living world from the world of the dead. Morgana went to the Pool of Nemhain and cast the coin into the lake’s depths, forcing Sir Lancelot’s spirit into the living world.
Morgana used the shade of Lancelot to come between Arthur and Gwen, who were preparing to get married. Lancelot succeeded with the help of a magical bracelet, and Gwen was banished from Camelot. Under Morgana’s orders, the shade of Lancelot then killed himself and Merlin was able to free his soul with the spell "Grið fæstne mid þisse tintregian sawle", allowing him to rest peacefully (Lancelot du Lac).