Domino is a mysterious group introduced in the season one finale. Not much is known about this group.
In speculation the flag of this mysterious group: one domino falling about to send the others crashing down, could mean the fall of the monarchy designed by the anti-monarchs.
In Season 2 Episode 1 Helena recognizes the symbol is burning in their garden. When her mother (The Grand Duchess) comes along Helena asks her, if they're (Domino) responsible for the death of Simon. The Duchess answers: "Of course they did. We kill one of them, they kill two of us. The questions is ... What do you plan to do about it?", she leaves with these words.