Nostradamus Enigmas are puzzles scattered throughout Paris, with each quest starting point represented by a scroll icon. Each Enimga quest involves using a riddle, given by the quest starting position, to find symbols that appear in Paris on the ground, walls, statues, and so on by using Eagle Vision.
To begin an Enigma puzzle you simply need to head to the starting location and interact with the symbol, marked with a quest accept. Once accepted you’ll receive the first riddle that indicates a location somewhere in Paris, usually within a limited area the size of a couple districts. Using the riddle you must find the location and use Eagle Vision to reveal the symbol and interact with it for the next riddle. This will continue until you receive the final riddle and final symbol, in which case you will complete the mission.
Once you complete all Enigma’s you can unlock the gates in the basement of the Cafe Theatre to gain Thomas De Cerneillion’s armor.