The disappearance of Ellena Lawmaker

After having gone to an invite-only location, whatever that meant exactly, Ellena had been missing for a year. Her sister, Delilah, had to take care of herself completely since then.
Ellena told her not to follow her -- But Delilah inevitably did when she, herself, received a note of the monster who might be responsible for her sister's disappearance; especially as the promise was made she could see her sister once more. She couldn't refuse.