The Grimoire of Essences* is a new system we've introduced in NosReaper that allows players to "acquire" new stats from bosses. When you defeat a boss, you have a very low chance of obtaining its essence (0.1%), which will allow you to set that essence into the grimoire and obtain its stats.
You can buy your grimoire from Grimoire Shop (Max 1 x Character)

You can enhance your grimoire in two ways, and each will yield different results.
The first would be with the Grimoire Upgrader* which will upgrade the grimoire's level. With this, you'll find that the effects of the essences become stronger, with +0 being the weakest and +10 being the most powerful level.
Lastly, but not least, we have the Grimoire Rarify* which allows us to increase the quality of our grimoire, enabling us to set more essences, meaning more different stats.

You can obtain these items by defeating the World Boss Nidhogg or Subzero Assault, whose schedule you can find here: Schedules
You can also acquire the items from Glacernon raids or craft them using Eternal Ice* at the Glacernon Craft NPC, located in Citadel.