This achievement is quite difficult to unlock if you don't know what to do. The riddles are not always entirely clear, and the hints provided in-game do not always help, so hopefully this guide will direct you to the location of each symbol without much hassle.
There are seven series of riddles that you have to complete to unlock the Sword of Suger. Two are done as a part of the main story, and you will have to progress that far to be able to complete the rest.
The missions are progressed by finding symbols located on the walls across Franciade. You must activate Eagle Vision to locate them, and you must solve each riddle in order. The separate missions can be attempted in any order. Once you know the location of the solution to a riddle, interact with it and you will be given the next riddle.I - Nativitatis et Mortis
The starting point for this series can be found on top of a church in the Windmill district, roughly central on the map. 1)
From the starting point, get to ground level and make your way to the other side of the church (facing south-west). There will be several guards around, so I suggest dealing with them. Looking at the church, you will see the doors, and a water bowl to the left. The symbol can be found behind the bowl.2)
You now want to head north. There is a building that you visit during the main story a couple of times, which has red walls. It's located in the Cemetery district (tough to give you directions via text; for more info, consult the first video below - 1:00
). There is a small courtyard at the north of the building, and the symbol can be found on that wall.3)
For the final one, fast travel to the sync point in the Cemetery district. Dive into the bale of hay below, and jump over the graveyard wall just to the north. There will be a tree stump right in front of you, and the symbol is located on it.II - Morbum
The starting point is on a tower on the outer wall, at the far north-east of the city.1)
Jump down from the top of the tower and head north. You will find small covered area, with several guards in front. Deal with the guards, and interact with the symbol located on the wooden wall.2)
Head south from the first location, and activate Eagle Vision. You should see the symbol on a door to the west.3)
Head south once again. The final symbol can be found on a wall at the end of a short alley.III - Diabolus
Upon completing story mission 3, you will have to complete this series of riddles. Simply follow the green markers and you will have no issues.IV - Natura
The starting point can be found roughly halfway up the west side of the restricted area in the Windmill district (the far south-west corner of the map)1)
For this one, head north-east to the windmill itself. It might help to kill some guards along the way. There are actually two symbols located around the windmill; you want the one that is more on the north side.2)
For the second one, head to the far south-west corner of the map. There will be a campfire and some stones. The symbol can be found on one of the stones.3)
Head north from the last symbol, along the western edge of the restricted area. About halfway up you will find a well in front of a wall. The symbol is on this wall.V - Crux
Upon completing story mission 3, you will have to complete this series of riddles. Simply follow the green markers and you will have no issues.VI - Noctis
The starting point for this series of riddles is located on a rooftop near the center of the map in the Basilica district.1)
From the start point, look south-east towards the large crescent-shaped building. If you have equipment that extends the range of your Eagle Vision, you may be able to see it from the roof. If not, then jump down and head in the direction I mentioned. You will have to stand on a pile of wood to interact with it.2)
From the first symbol, look in a westerly direction. There is a road running in a west-north-west direction away from you. If you look on the main map, you should see a small circle in a triangular courtyard (at 1:00
in the second video. This is your destination. The circle represents a fountain, and next to it is a tree. The symbol is on the tree.3)
The final symbol can be found in the cemetery in the Cemetery district. Fast travel to the church, and head towards the northern-most alarm bell in the restricted area. The rock with the symbol on can be found there.VII - Dies
The final set of riddles starts in the graveyard of the Basilica.1)
From the start point, look towards the Basilica, and you will see a large circular window. Climb up the side, and the symbol can be interacted with by standing on the balcony in front. You may be able to see the back of it, depending on the route you took up, but must activate it from the front.2)
Begin making your way down the Basilica, and head along the west wall of the building. Once you are out of the graveyard, activate Eagle Vision, and you will see the symbol on the wall near the front of the building.3)
Head around to the front of the Basilica, and climb up the doors. There is an open window directly above. Climb in, head up the short set of steps on either side, then up the wooden stairs into the library in the Basilica. Activate Eagle Vision and you will see the final symbol on the floor next to the holy water font. Activate it to solve the final riddle.
Once you have complete all seven series of riddles, follow the scroll icon to the door in the wall in the next room. Interact with it, and the door will open, revealing the Sword of Suger. Your achievement will unlock!
The videos below show you exactly where to go for each of the riddles. Hopefully you will find them useful.