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?