Death magic was a branch of dark side magic practiced by the Presagers of Hakotei.
The Presagers used living sacrifices, specifically their fresh blood, to see visions of the future. In order to use their magic, the Presagers needed a nexus for the dark side of the Force, such as those found on Hakotei or Asusto. They also believed that a "conduit" would come along that would allow them to use their magic to see every possible future. When he came to Asusto with Jedi Master Lene Kostana and her Padawan Sifo-Dyas, the Presagers believed a young Dooku was such a conduit. As a result of their ritual, he saw many potential futures, including futures in which the Jedi conquered the Galactic Republic. He also saw a vision of his future Master, Darth Sidious.
Death magic first appeared in the 2019 audio drama Dooku: Jedi Lost, which was written by Cavan Scott.