The form was lifeless and inert, but it seemed to Smith as he gazed that there still lingered a lurid spark of vitality, some faint sign of consciousness in the little eyes which lurked in depths of the hollow sockets.

-Sir Arthur Conan Doyle, Lot No. 249

The undead monsters Van Richten dubbed the ancient dead are more widely recognized as mummies, but a shambling, bandaged monstrosity is only one of the appearances these fearsome creatures might take. Any culture that preserves the bodies and venerates the memories of the deceased might unknowingly harbor the ancient dead in its burial ground, and these creatures don't kindly suffer the intrusions of the living.

Ranks

Like ghosts, the ancient dead are separated into ranks. The higher a mummy's rank, the more powerful it is. This is roughly determined by how powerful the mummy was in life, how venerated it was in life and after death, and how much care and expense went into its preservation and burial. Rank one mummies were usually servants mummified with their master, while rank five mummies were usually rulers of entire nations.

Ancient dead possess enhanced powers compared to the default mummies many adventurers know. They can also develop salient powers as well, making each encounter a unique one.