There are a total of 7 Suger’s Legacy Riddles in the “Dead Kings” DLC of Assassin’s Creed Unity. Completing all of them will earn you the “Defender Of Franciade” trophy / achievement and unlock a very powerful sword called Eagle of Suger.
Suger’s Legacy Riddles are very similar to the Nostradamus Enigmas in the main game. In them you have to find hidden symbols in the town of Franciade. The first two Suger Legacy quests are part of the story and cannot be missed. They take place after completing Sequence 13, Memory 3 and unlock the starting points of the other 5 quests on your world map.
To play the DLC and access the town of Franciade you have to use the carriage near the Café Théâtre in Paris.
If you only need help with a specific riddle you can click the time-links below: