The Grim Grimoire is an informative item for players, allowing them to access evolutions and unions they've already unlocked. This will help many people from having to tab out of the game to remind themselves about certain builds or even if they have certain items unlocked.
But where do you find it? As with most hidden items or characters, you're going to have to travel a lot in one direction.
Where to Unlock the Grim Grimoire in Vampire Survivors
Players will need to travel to the Inlaid Library, the second stage in the game. There should be a green arrow pointing toward the west. Head that direction until you come across a purple tome with a red bookmark. The Grimoire will be obtained.
The only thing missing is it doesn't show you every weapon that you haven't unlocked yet. So you'll still need to use third-party sources to find new items or builds. But it should help in the long run.
Once you find it, a new button appears at the bottom right of the pause menu called "Open Grimoire." Once you do, the sub menu will appear showing you everything that you have unlocked.