Domino is first witnessed by the palace gates with its symbol on a waving flag following King Simon's death. Queen Helena seems to be familiar with the symbol and when it is burned into the palace lawn, Helena discusses it with her mother who relays she's sure they were behind both Simon and Robert's murders:
We kill one of them, they kill two of us. The questions is ... What do you plan to do about it?
It is eventually revealed that the orchestrator of Domino is a man named Jeffrey Stuart, who dedicated his life to punishing The Royal Family for the death of his sister, Dominique. The Grand Duchess had Dominique murdered as she was the love of Simon's life whom he planned to marry, and Alexandra wanted her daughter to be the Queen of England.