Everything

From a story standpoint, the necromancer typically gathers remains from the dead person’s body, “fetters” from its old life, or possibly people, locations, or items involved in its death. In Wraith: The Oblivion, these fetters link the ghost to its previous existence, and give it a tie to the living world. In Mage terms, such artifacts count as “personal items” on the Magickal Difficulty Modifiers chart. Regardless of the system involved, those fetters give the mage some power over the ghost. And while a sympathetic mystic could offer to protect or preserve the fetter, a ruthless one might threaten to destroy it unless the spirit does what she wants.