Discover the Assassin's Tomb inside the catacombs under Santa Maria Novella.
This is story related and cannot be missed. This will unlock upon completion of the memory ‘Novella’s Secret’ during DNA Sequence 4.
This tomb is located along the eastern wall of Santa Maria Novella in the southwest part of Florence. You can access it in two ways; 1) directly kill the guards standing at the entry gate or 2) jump from an adjacent rooftop into the courtyard. You can also draw them away using Courtesans. The skull “lever” is in an alcove along the short wall.
The goal of this memory is to infiltrate the catacombs underneath the church to spy on the Templars’ meeting. It starts off with a jumping sequence to reach a ledge to pull a lever to progress further into the tomb. Next, you must find a way to open a door and kill the patrolling guards. After chasing the final guard, climb up and activate the skull lever. A wall will move prompting a cutscene of the meeting. Eavesdrop on it and listen to what they are planning. When it is over, you are instructed to open the sarcophagus at the end of the passageway which awards you with one of the Seals of the Assassin’s. Exit to the right and the achievement will unlock.
@zombeh95 You get it through natural game progression. I believe it happens during a mission in sequence 4-5. You will have to navigate through this underground tunnel and you will come across the first one in there. Its a freebe really, Rather easy to get.