How can a creature that feeds on energy not grow more powerful?

Question

Elves are magical creatures that use Mana as a source of power for their spells. Individuals are born with different amounts, which increases as they age and will determine their potential of magical power. Mana constantly regenerates in the person after it is used in spells. Elven maidens with large reserves of Mana usually produce powerful children. Mana is transfered from the mother to the child through the umbilical cord during pregnancy. Under normal circumstances, the baby feeds of this mana to grow over a period of nine months.

There are times in which the developing child sucks the mother's life force faster than she can regenerate it. In these rare cases, the maiden gives birth to a dark elf. These creatures completely suck their parent dry of Mana, turning them into a husk. They contain higher than normal amounts of Mana, continuing to grow in power as they get older.

These creatures roam the earth as Mana empowered abominations that feed on others. They drain the Mana from victims by simply being in the vicinity. Elves become weaker the closer they are to a dark elf, and they will continue to waste away the longer they are in their presence. Dark elves cannot be defeated by spells due to them feeding on this energy.

However, the dark elf does not grow any stronger from the Mana that it takes in. Regardless of how much it feeds, the Mana it drains from others does not increase its own supply, preventing it from becoming more powerful. How can this be the case?

Answer 1

The dark elf itself does not feed on mana. Its parasite does.

The event that leads to the formation of a dark elf is not the fault of the fetus, or the mother. It is caused by the intervention of a dark force in a dimension adjacent to our own. The normal interaction between fetus and mother causes a small irregularity in the power structure of our own dimension and a dark entity can take advantage of this, ripping it larger and turning the fetus into the equivalent of a mana black hole. Mana is pulled towards the fetus, and later the baby dark elf and through its body the mana leaves our dimension entirely.

The dark elf does not benefit directly from this. It is not in control of the process. Mana sucked from the environment into the dark elf is actually sucked into the rift within it, from there feeding the dark entity that has parasitized it.

That does not mean dark elves are good. The elves know when a baby is a dark elf and know what it will become. In most circumstances the baby is put down as soon as this becomes evident. The entity which hopes to use the dark elf as a conduit knows this too and tries to keep the baby safe so it can grow up. It may cause it to move off into the wilderness. It can communicate with the dark elf, initially via thoughts and impulses and then as more sophisticated communications. It tries to keep the baby from dying. In one sense, the dark entity raises this baby. Some dark elves, isolated for their whole lives, cannot distinguish the urges and voices they perceive from this entity from their own inner discourse.

There are bands of dark elves who find each other, or take up residence with other creatures that because of their natures are not harmed by the proximity of the dark elves. In the company of other sentient creatures, these dark elves can gain perspective and might come to understand what they are. This might or might not be in the interest of the entities that use them as conduits.

Answer 2

The mutation makes their Mana incompatible with the Mana they absorb. They are in a position of a human endlessly eating grass -- it does not nourish them.

Answer 3

How you described mana, seems like any other biological substance used by the body to say alive, like blood for example.

Blood is Hella good for you, without it you literally die. Blood, just like mana is transfered to unborn children so they can grow, and just like mana It can be transfered or in that case "stolen"

However, too much blood can make your organs collapse. Eating blood can make your liver fail because it has too much Iron and vitamins... Yeah you can actually die from Vitamin A excess, no joke.

You could in theory have more blood than needed inside your veins, but it's not gonna be good for you.

Why? I mean blood gives your cells oxygen, and good oxigenation makes people stronger, faster and probably smarter.

But there's just no room in the veins for that much blood, and the heart can't pump all of it without a few strokes getting in the way.

Your mana is just a secondary blood system.

Answer 4

Extreme metabolism

Your dark elves are beings surrounded by their own misfortune. Due to a mutation caused by a relatively rare recessive gene, they possess a much higher mana abortion capability, being able to simply absorb mana from everything around them. Sadly, this comes at a price: they have a metabolism much faster than a normal elf's. The usual symptoms of a high metabolism for humans are: higher temperatures than normal, problems gaining weight, increased urine volume and, most importantly and especially in more extreme cases, insatiable hunger. Your dark elves have a similar condition: while they produce much larger amounts of mana than a normal elf, their bodies have such a high metabolism that they need to use mana to fuel themselves with the energy they need. That means they consume equally large amounts just to help keep their bodies running and avoid loosing weight, a condition that can already be seen before birth, as the baby drains an amount of mana superior to the mother's production capacity in order to sustain itself.Their metabolism, while always being higher than any normal elf, vary according to their intake, meaning even incredibly large amounts barely do anything, as the abundant amount of food causes positive feedback on their bodies and increase their metabolic rate further (similar to how exercising increases our metabolism), which could mean for a short while they might have slightly increased senses and reaction speed, but that's up to you whether they just burn through their reserves faster or if feeding a lot actually grants them some kind of short term buff.

In other words, while they might seem like voracious abominations on the outside, deep down they're just elves with a rare condition, who can never truly satiate their hunger and just can't quite stop feeding on everything around them for as long as they live.