AbeTheFlyingCarrot
Once, a race of monsters lived far below the Earth's surface. Intent on returning to the world above ground, they swore to slaughter every human to enter their prison. However, when a human child nearly fell to their death, even the Underground's rulers could not bring themselves to harm a young, innocent creature. The king and queen of the monsters took the child into their home, welcoming her as a member of their family.
After years together, the human child, along with her new brother, the biological son of the king and queen, disappeared without a trace. The Underground mourned them without proof of their deaths. The king and queen ceased to age, something only possible with their children dead.
Broken-spirited, the king and queen decided that the next human to enter their realm would be killed, and a great warrior would be chosen to absorb their powerful soul, cross the barrier, and gather enough souls to free monsters forever.