It was a blood sacrifice.
As customary of House Asher, families would give up their first-born son or daughter to
Yusei, a powerful eldritch horror of Spider Lily Halls, Yusei's territory within the Dark City. The ritual was elaborate, and usually ended up with the same fate. Death.
The parents always had a hope, though, that their child would be
chosen.
Most often, as it was the past ten years, Yusei would take the offerings and leave not a trace, consuming the child and all the years of life prescribed to it. However, when little Reina was presented at the alter, instead of vanishing like all the rest… a prismal light descended on her. The little albino opened her muddy brown eyes, and in a flash, red light burst forth from the depths of them, permanently changing them. The mark of Yusei.
As the child grew, she spent much of her time in Yusei's charge. As her diet changed from food to devouring years, much like her master, Yusei taught her how to capture and eat weaker monsters in the Dark City, and tame the stronger ones.
Yusei also helped Reina navigate the labyrinth, leading her down to rooms that would allow her to summon the monsters she had tamed, and subjugate the foolhardy with the powers of obscurity.
Being chosen by Yusei put Reina in the reigning position of House Asher, and elevated her family. Yes, there are many who would eliminate her within the Family, hoping Yusei would be forced to choose
their first born. Others still play political games, hoping to make Reina their puppet.
House Asher, of course, is steeped in the obscurity aspect, often using spells that prey on the unaware and curse those who look at the House the wrong way. The families within the House are cult-like, seeking to be favored by Yusei and his subjects.
No one quite realizes the toll having your soul connected to Yusei brings. Not only did Yusei devour half of Reina's lifespan, but Reina lives with varying degrees of madness she is trying to find a cure.
Yusei's goal, of course, is to devour the lifespans of Reina's enemies, and pushes her for military power and domain expansion.
Reina, being at the pinnacle of it all, has enemies both inside and outside of House Asher. But under the guidance of Yusei, she will lead the House to greatness, making it the greatest powerhouse in the city.