The Essence of Perfection is a masterwork crafting material. When you equip it in the masterwork slot during crafting it has a 100% chance to grant you a +10% boost to all your stats.
The other versions of this masterwork material, like for example Dragon’s Tooth will only give you a 40% chance to grant +10% stats, or (10% chance from Ironbark, see here for reference).
The Essence of Perfection can actually be found from doing the main story mission Here Lies The Abyss when you reach the Fade level. You’ll find the loot towards the end of the level.
Pick it up and it will contain the Essence of Perfection.
This loot drop is actually a reward from one of the quest in map called “Fear Of Dreamers” (the one which requires you to find five fears). It’s not a hard to miss quest so you’ll always get the essence of perfection.