"Essences are used both in crafting magical items, as well reagents for spells. Elemental essences are dropped by their respective guardians"(bosses)", and all essences can also be crafted using the Essence Refiner."
Crafting/obtaining EditLightning Essence on top, Ice on the left, Nature on the Right, Arcane on the bottom, and Arcane Ash in the center.
The high essence core can also be dropped by some bosses.
The usage of the high essence core is only for crafting special items or rarely(citation needed) spells.