It has an AC of 15, and is immune to all types of damage except Acid, Radiant, or Psychic. The spirits inside the book communicate with their host telepathically, and use the same senses of their host. Without a host, they can feel the passing of time, but have no sense of its surroundings.
To regular eyes, this book has the appearance of a regular spell book. It has a worn brown leather cover and two leather straps sealing it shut. A Successful Detect Maic spell, Divine Sense, or Truesight, it appears to be dripping with dark magic. To magic users or magical creatures, it's like standing next to sewage. The book is covered in glowing runes and sigils. It has a large eye on the cover that will stare down anyone who can see the book's true form like it can see their soul. The book is wrapped in thick chains and padlocks locking it shut.
There are four other of these books spread across the realms - the original mages to lock the spirits away into them split the books. If all the books were in the same place, the spirits may become powerful enough to escape. Legends of these tomes are known across the realms both to those who wish to protect them and to those who wish to use them for nefarious purposes. This book and its siblings have been kept under the care of powerful wizards and mages for centuries, under layers and layers of protective spells, to keep the dark magic and spirits sealed inside. If someone opened it, it would release the spirits into the "host". The spirits and book grant their host spells and inhuman abilities but take their very soul and life force in return. The spirits believe in equality, give and take. They can be trusted to fulfill a bargain, but not to strike a fair bargain. A host must carry the book at all times, or risk the wrath of the spirits inside.
Despite the layers of protective spells, the book will open without resistance. The writing inside appears to be unintelligible words in an ancient language they can't recognize or understand, in a pen so dark it looks like blood. A player must pass an arcana check of 15 to read it. If they are successful, they become the spirits' host, leaving them physically unharmed but fundamentally changed. They can now see souls, dead or alive, and any individual's fate, which hangs over their head like a grim, garbled omen. Occasionally, they may speak in a language, the same one the book's script is written in, that no living being recognizes or understands, because it doesn't truly exist.
Notes: Vulnerability: Psychic, Sense: Truesight, Proficiency: Insight, Proficiency: Perception, Feat: Magic Initiate, Utility, Cursed, Sentient, Symbiotic