Essence, from what I know, is supposed to be required for certain spells, and to make cities. It also is only capable of being raised by leveling up.

From most all other RPGs, levelling up would probably be harder and harder as the game goes on. As well as later in the game, the fighting gets more dangerous, so you might now want to put your soverign in danger. And, thus, essence is a super rare valuable resource, that is hard to get more of.

I, as a player, HATE THAT. I see that, unless I want to risk my soverign and an instant game over, I can never get more. And as a turn based game, games tend to last a long time. So what's my initial instinct? To hoard all of the essence, and never ever spend any of it. Which, of course, means that some cool spells I'd never actually get to use except in the first five minutes when I'm assured to win.

My suggestion: Implement some way to use a renewable resource(mana, time, or both) to create essence. It doesn't have to be particularly effective, just as long as there's a way, avaliable middle of the game or so at the longest. That would help the hoarding reflex a lot.

Alternatively, simply have it an 'investment' that you can take back at any time. Investing into an item which you can take back(though, maybe needing to destroy the actual item to take it back), taking your power back out of the land causing it to slowly revert, your essence coming back when one of your champions die/you choose to retake it. Similar to concept to units in RTS games taking up 'command points', except with powerful ongoing spells and items. By having the choice to get your essence back to remove an effect(even if it requires you to go kill something), the hoarding reflex is gone.