This is a story related achievement and is the final part of the 'glitched memory'. It acts as a prologue to the main game's story line, and will automatically start after you've watched a few cut scenes/credits following the final mission of sequence 6.
Those of you who have played AC: Unity will recognise that this mission is a scene from that game, but from Shay's perspective.Quick Summary
There are 3 steps to completing this mission and to avoid spoilers I'll only provide the key details:
Step 1) Infiltrate a building without building detected. You'll first try an entranceway ahead of you. It will shut. You then need to back-up and try a window instead. A helpful green icon will appear with the word 'infiltrate' above it - telling you where to go.
Step 2) Once inside, stealth your way through the building without being detected. There are a lot of guards and you have to make your way through about 6 rooms. Fortunately there is a party going on so there are plenty of groups of people to blend in to.
Step 3) Assassinate your target without being detected. He breaks off on his own fairly quickly so you should be able to sneak up on him while remaining blended and then get him with your hidden blade.Video Guide
If you would like a video guide, please see the below. It's not my video and credit goes to the creator of it: XCV //