You are only 15 but your parents drink everyday, do drugs, and they hit you for no reason. They'll beat you until your unconscious and laugh about it, calling you weak. They'll lock you in your room for days and weeks without any food just for 'fun.'

Nobody knows how Slenderman knows, he just does. You were sleeping in your room, scars and bruises all over your body. You didn't have blankets or even anything. Just a mattress. Slenderman snuck into your house that night.

He picked you up and left with you. After quite a bit, you woke up to loud voices and people chatting. You saw you were in someone's bed, and it actually had sheets and blankets and other stuff to make it look lively.

You were in the Slendermansion.