A Bygone is an object imbued with the strange abilities that occasionally resurface in the Chronicles of Darkness. While similar items may exist, each is its own item, each tells its own story. Certain phenomena occur with some frequency. So while characters can research a desired relic or effect, the examples they find might not line up congruently with the relics they read or heard about.
Most bygones are very dangerous and require special treatment to be handled correctly, which can lead to problems if the finder is already accustomed to certain mystical objects (like Fetishes for the Uratha or Vessels for the Arisen). Bygones cannot be replicated and their abilities are often lost once they have been used.
- The God-Machine Chronicle, p.243