The little Zoldycks deal with the devil

Prologue

8 years ago:

The sound of screaming filled the halls as Kikyo was on the bed trying to give birth to her baby. The whole family was there, Silva, Illumi, Zeno, Maha, Milluki, Kalluto and Killua were waiting outside the room as the maids were in there helping the mother.

'Otou-san, why is okaa-san screaming?' The four year old Killua asks curiously.

'She's giving birth to your little brother,' Silva replied.

The little killua was in Illumi's arms and snuggled into him as he was tired and started drifting off.
The maid then opened the door saying the family could come in now, then politely bowed as her masters entered.
Kikyo was on the bed holding two bundles of blankets, one pink and one blue, close to her face.

'Twins? Have you decided what to name them?' Asked Silva.

'The boy is Alluka and the girl is Seiko.'

Three years later:

'She's useless,'Kikyo said to Silva one day at a meeting, 'Alluka is already proving his potential. Killua is the next heir and has already killed over one hundred people, yet Seiko is... is......  she is not needed by this family.'

'How about we give her to Illumi to train her? She might get powerful later on.'
'That might work.'
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'AH!' You yelped as you felt your back make contact with a hard, brick wall.
'This is going to be interesting, Seiko, now stay there like the weak little girl you are,' Illumi spat with disgust and left you in your 'new room' crying on cement floor until you fell asleep.

The next day, Illumi came into the room around  5 minutes after she woke up- with a large needle in his hand.
As the girl was still sleepy, he attached her to chains (like killua's but on the floor) and slapped her cheek. 'Wake up weakling.'
'Mhmn... ow.....' her bright pink eyes adjusted to the light, well there wasn't much, and lifted her head slightly. You realised your position and started to resist slightly, knowing what was about to happen.

'Stop moving.'

He then inserted the needle into your neck, pushing past the muscles and bones which made you scream in pain. Illumi then took to the needle out slowly, twisting it back and forth as the veins got tangled around it- this made it ask the more painful for Seiko until she passed out from the pain and exhaustion from screaming. A small stream of blood ran down your neck when Illumi took the needle out, the contents were all running throughout your body, burning your insides.

This torture continues every day (Illumi never spent too long on missions since he is very skilled) and Seiko was always close to death.
She was hardly given enough food and water to live and her health was declining rapidly. The cell that she was in wasn't very sanitary as some of Illumi's 'medicines'- as he likes to call it-were scattered on the floor after being dropped. Also the countless amount of red blood on the walls and floor were hardly unnoticeable, there was almost always some fresh blood on the surfaces of the cell every day.
Seiko continued to do her best to live and keep hold onto life throughout her childhood.

This continued on for years.
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Five years later:

'Ah, ..... shinitai (I wanna die) .... shinitai..... shinitai... help me........'
However your wishes weren't heard as Illumi came in with his emotionless face and holding a needle with a green, bubbling fluid inside it.
'Seiko, you look half-dead. That's bad. I hope you're ready for the punishment.'
Without waiting for an answer, Illumi shoved the needle into your shoulder which hurt a lot, but you couldn't even scream anymore from lack of energy, so you just saw black and passed out.
Every day, you are only getting the bare minimal to live, a small piece of bread and one cup of water for breakfast and dinner- both shoved down your throat which made you choke a lot.