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Under the ruins of the old castle lies the Great Evil from the past ages, sleeping lightly, waiting for the moment to appear and rain destruction on the earth. For now, it has no possibilities for awakening - it seems to be too weak, but day by day it grows stronger and could eventually shrug this sleep off.

Brave heroes, ready to battle, come into the castle - here are many reasons for them to do this, from just looting the abandoned keeps in the ruins, to patrolling the woods to stop bandit raids, etc. But they must be very, very careful, since having any of them dead will fuel the power of the Great Evil and shorten the time until its awakening.

And here's a problem: why only the heroes' deaths are counted here? They may kill a lot of creatures - but that doesn't harm in any way, the Evil feeds only on the "good guys". What could make the difference?

Some ideas I'm thinking of:

• Many of the "bad guys" are soulless (or even mindless, like giant jellyfish or undead). Great Evil gains strength only from human souls, it has no use for such garbage. But what about the mentioned bandits? They are bad, but not so evil to call them soulless.


• Heroes carry some kind of internal strength, which is not possessed by the "bad guys" - that's the reason they can win many battles in a row, but that's also why they are of interest for Evil. But I don't want to handwave it like this and can't yet think of some more detail.

Disclaimer: This question is based on the game mechanics (the story, in fact, is planned to be like "extended novellization", based of the game plot but adding a lot of details). I'm deliberately not including game's name in the question, so it doesn't influence the answers.

You could make those creatures in the dungeon be somehow the effect of the evil being underneath. Its dark aura create monsters that feed on the world to give energy to the evil and help him waking up.
As those monsters already are a part of the evil, killing those don't give new energy to the evil.
About the thieves, you could make that humans sharing lot of resemblance with the evil will make their energy getting eaten by the evil, and become part of it (The more they are evil, the more energy will be taken and replaced with evil's energy. Evil energy will try to make the human turn even more evil day by day to eat him entirely). As their energy is already inside the stomach of the evil, killing them won't increase the power of the great evil. As the bandits will eventually end up being entirely consumed, killing them will give energy to the great evil, but the great evil would have ended up getting that energy one day anyway. You could find a way to make heroes capture bandits and try to save them from evil instead of killing them.

If a hero gets killed tho, the great evil can eat their energy.
This way, the great evil won't simply grow stronger day by day magically, it will actually use the monsters to get stronger, making one more reason for heroes to feel like they should kill the monsters and not let the bad guys go rampage.

Only human deaths the Great Evil causes will help fuel it.

The deaths could be direct: someone gets too close to a piece of the GE and it has just enough strength to dispatch them.

Or the deaths could be indirect: someone falls for a boobytrap the GE set up before its great sleep (which includes every bit of those grounds and woods...any death there counts).

Either way, it explains why, at least for now, the only deaths that make a difference to the GE are those in or around the castle grounds.

The GE does not wish to kill off people who are evil or at least have some evil potential (like a bandit) because that would diminish the evil it puts out into the world.

If an evil person is killed by accident (or boobytrap), the GE does get fuel from that death, but it also diminishes the evil outside of itself, so the net effect is zero for the GE.

If a righteous person is killed, the GE gets fuel, period. There is no evil lost. The fact that the world is a little bit diminished by the loss of this person brings nothing but joy to the GE.

Note: if your world has sentient creatures other than humans, they can be included in this. Any species that has the capacity to choose good or evil can affect the GE.

It is a gamification of the kill-a-hero task. The great Evils are a group of ancient beings in charge of keeping the world clean from heroes. They tend to be lazy, so, as an incentive, every few heroes that they kill they get awarded by the Supreme Law with the ability to spawn a new creature of their choice.

The Supreme Law is known to humans as the Law of Mediocrity, according to which the world is best inhabited by perfectly mediocre beings. Heroes being exceptional and outstanding are very unwelcome in such paradigm.

The way in which the Supreme Law awards such prizes is akin to the way points are collected in a supermarket. Each Supreme Being has a huge box with sections in it, each section is about the size of a hero's head. In fact, the heroes are decapitated, and their head is placed in the corresponding section. Every few heroes, the Evil being is awarded a prize, usually in the form of a creature, as we said before. When all the boxes are filled, the Supreme Being can claim the final prize, which is called the Awakening, and it consists of a one-being trip to the realms of the living to check that the Supreme Law indeed reigns undisturbed.

The awards and the final prize are provided by the extra-dimensional minions of the Supreme Law. If you are an Evil Being you should hurry up, because this offer will be ending soon.

It's a matter of strength.

Some mortals have fought and gathered strength and are the paragons of their kind. They can fight or sneak or spell their way through hosts of monsters and enemies to go fight the monster. They have powerful magical weapons, blessings from gods and spirits and archmages, they have alchemical concoctions strong enough to restore them from death's door, they have imbued themselves with the blood of their foes to gain greater strength.

They make a decent snack. Random monsters? Barely an appetizer. If they ate every monster within the castle it would barely be enough to cover their daily calorific requirements.

The more powerful monsters within the castle are net drains. They use the ambient energy of the monster to fuel their inhuman might. If slain, their power simply returns to the monster, but when it's powerful enough to awaken it would seize their power anyway.