Supernatural Index (5e Equipment)
From D&D Wiki
A Supernatural Index is a book. A book kept by all hunters of supernatural beings it contains useful information on all the creatures you have deemed worthy to be entered into its pages. These books are usually kept by Celestial Hunters and contain information on creatures whose information have been forgotten. A person trained in the use of these confusing books can glean information about all sorts of creatures. If you are within 10 feet of a magical creature or a creature capable of using magic, you can use a bonus action to make a History skill check to jot down information about the creature in your Supernatural Index. On a successful skill check, you now know the creature's Maximum Hit Points, its AC, its base walking speed, its Actions, Reactions, and Legendary Actions.