What is Essence?
Essence is a special resource that you can only get from Event rewards. It can be used to unlock spells that you really need for specific decks. However keep in mind that unlocking spells for a hero makes the next spell more expensive. Therefore make sure not to unlock spells with Essence too often.
How do I use Essence?
Once you have enough Essence you get the unlock spell option in the spell collection of each hero.
The starting unlock costs are 100 for the first basic or advanced spell, 150 for elite and 300 for ultimate. Each time you unlock a spell for a hero it increases the costs for that hero.