The grim reaper is just a conscious representation of Death. Death itself is a form of energy that is connected intimately with life itself. Representations of death can be worked with and gnosis on death can be reached through necromancy. In necromancy, you learn to walk with death and use its power to hurt and to heal. Some manifestations of the death one may work with are: Qayin, Santisma Muerte, Senor La Muerte (a mask of Qayin, according to my gnosis), Mictlantehcuhtli, Hela, Thanatos, Hades and Persephone, Nergal and Ershkigal, Belphegor, Bune, Baron Samedi and Maman Brígitte, Ninigizzida, Exu, Baron Cimitiérre, Baron La Croix, The Spirit of Saturn, certain masks of Asmodeus, Samael, Azrael, Osiris, Anubis, Nephtys (probably misspelled that), Odhinn, Met Kalfú, Yama, Rahu, Shiva, Yam Nahar, Kali, Parvati, Amiahatzan, Qalmana, The Spirit of Saturn, Nidhogg the Death Serpent, and many others. These spirits each have a piece of death gnosis to bestow and these all differ but are all equally true. I would say that the entities closest to the traditional western grim reaper are Qayin, Senor La Muerte, Qalmana, and Santisma Muerté.