The entire game of Eternal Darkness: Sanity's Requiem revolves around the story of the Tome itself. With the exception of Pious Augustus (who later becomes the main antagonist), each character will find the Tome early on in their level. Visually, the Tome acts as the menu system, with each page being a different part of the in-game menu (map, inventory, and so on). Narratively, the Tome fulfils two important functions:
Firstly, the Tome contains a number of chapters, which chart the story of the playable characters themselves. The primary character, Alex Roivas, must explore her ancestral home and find the lost chapter pages, uncovering the centuries-long battle against the Darkness as she does so. Upon reading each chapter page, the player is taken into the next part of the story, gaining control over the chapter's protagonist as a playable character. In each case, these characters also find the Tome, and discover the story of all those who have gone before them.
Secondly, possession of the Tome gives each character access to the Ancients' runic magick. Runes and spells are stored within the Tome, allowing each character to access the spells that previous characters have discovered, and also to add their own.
In addition, with the exception of members of the Roivas family as well as Pious Augustus and Ellia (both of whom managed to get it under unknown circumstances offscreen), all Tome bearer characters at some point in their story are teleported to a otherworldly room with the floor lined with screaming faces, statues of deceased tome bearers and Pious Augustus, and a huge skeletal hand that holds the Tome of Eternal Darkness, requiring the characters touch it before being warped back to their reality. In the case of the Roivas family, they only need to locate the Tome inside the family mansion during their storylines (Maximillian Roivas behind a fireplace in the dining room, and Edward and Alexandra Roivas in a secret room behind a shelf in the library). A letter from Aaron Roivas to Max implies that the Tome of Eternal Darkness in the Roivas' possession was merely a copy of the real one, explaining its presence with them and not requiring them to warp to its holding place.
- The Tome has a number of unusual properties:
- 1. The Tome and its separated pages are written in a maddening, indecipherable scrawl. Only a select few are able to read it: no amount of cryptography or magick can decode it. These few are able to read the Tome regardless of their native language or their literacy level. Ellia, as a slave, is almost certainly illiterate and can still read it.
- 2. Most characters find the Tome in a special extradimensional space. It seems the Tome is able to return to this place on its own, or possibly exist in multiple space/time locations simultaneously.
- Although the tome acts as a menu, players do not need to find the book to access the menu. They do however, if they are to cast spells.
- At the end of each chapter after Pious', Alex starts with lower amounts of sanity.