Immortality slab

An immortality slab is a legendary artifact that holds the secrets of life and death, thus giving the reader the power of necromancy. Immortality slabs are always given to devout followers of a deity who became obsessed with prolonging their lifespan and who worship said deity. However, not just the recipients are limited to gaining the secrets upon. While the first receiver will store the item and presumably bar access to it, the slab can be taken (though usually through force, not diplomacy, trade etc.), even during worldgen. There are even instances of necromancers who learned their craft from the slab of their gods who then store the immortality slabs of other creatures in their towers, should they come in possession of them.