The Rite of the Revenant was an ancient ritual which could bring a dead person back to life usually as a lich (a spirit who possesses a host body which often decays over time) or as a revenant (the spirit is brought back to "unlife" in its own body or a close replica of it like a clone). However, the spell required powerful and talented people to cast it, and it also required a tremendous sacrifice from the casters because of its unholy nature.
Kagetsu Aurelac de Maar Sul planned to use the rite to bring back his deceased chief engineer Liosliath de Mont Hault to help him build airships in the Arawn Losstarot War but he died before he could begin the rite.
Drishnek used the rite in 1003 AE to summon the original Jemuel's spirit into the clone body of the Apprentice who was one of the Clones of Jemuel that had been raised by Dante Albrigant, banishing the clone's original spirit in the process.
Kareth d'Zarnagon used the rite (although he called it the Ritual of Malakhia) in 1004 AE to bring the twelve members of the Fire Lizards back to life as liches in the bodies of his acolytes with the cost of tearing off parts of his son Xerathas's soul in the process.
Persephone Barca and her accomplices used the rite in 1017 AE to bring back her mother Nyx Barca. She accomplished this through a complex ritual which involved Nyx's original, decaying body which she had stored for this very purpose.
The Mad Clergies used a variant of this ritual to try to steal back the goddesses Gaea and Heath from the grip of Death. Due to the interference of the traitor Breivik, their purpose was an utter failure, instead bringing the kitsune Inari Okami back to unlife, while the body that supposedly contained Heath was destroyed. The clergy paid for her resurrection with their lives.