Hexical essence are used to make Hexical Diamonds, which are essential in crafting pieces of the three Hexxit Gear Armor sets (Tribal, Thief, and Scale). One Hexical Diamond is needed for each Hexxit Gear armor piece.
The essence can be harvested from the Hexbiscus flower. It is a rare flower that can be found in many biomes such as plains, forests, and snow biomes. One can occasionally find Hexical Essence in chests in dungeons. It is also sometimes dropped by Infernal Mobs. It can also be found in the Twilight Forest.
Hexical Essence can be used to craft a Hexical Diamond, which can then be used to craft the above-mentioned Hexxit Gear armor sets.