SPOILERS for The Poison Garden by Alex Marwood

I read it to be that Romy killed Lucien by giving him more and more Vicodin. She was the evil one in this book, but that is not made clear until late in the book.

Romy had to complete these tasks for Uri (Lucien's oldest son) to be welcomed back.  Oh, and it was Uri who drugged the cult. Romy was the one who told them where to find the poison. 

Ilo killed Eden because Romy couldn't do it.  The cult was ready to welcome Romy back at that point, so she packed up Ilo and her Aunt Sarah (this shocked me!!) and went to the second property of the cult, the one that was under Uri's control.

The book ended with Romy plotting to kill Uri.

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